FUNCTIONAL INTERRELATIONS BETWEEN CONCHOLOGICAL and ANATOMICAL CHARACTERS in STYLOMMATOPHORA (MOLLUSCA, GASTROPODA) Anatoly A

Characters in Stylommatophora (Mollusca) 269

Сборник трудов Зоологического музея МГУ им. М.В. Ломоносова Archives of Zoological Museum of Lomonosov Moscow State University Том / Vol. 54 Cтр. / Pр. 269–297

FUNCTIONAL INTERRELATIONS BETWEEN CONCHOLOGICAL AND ANATOMICAL CHARACTERS IN STYLOMMATOPHORA (MOLLUSCA, GASTROPODA) Anatoly A. Schileyko Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences; [email protected]

Parameters of shells (shape, size) of stylommatophoran mollusks are determined by the history of the group, peculiarities of their food and con- ditions of life in different geological periods, as well as genetic nature. The history of Stylommatophora since the Paleozoic is briefl y discussed. It is assumed that one of the important factors determined the historical development of stylommatophorans were peculiarities of their food. The aperture is the main channel of communication of a mollusk with the outside world and any elements of the apertural structure are related, directly or indirectly, with the adaptation of animals to their environment. Therefore, special attention is paid in the article to the morphofunctional analysis of apertural structures. The main results of this analysis may be summarized by six basic conclusions. 1. The teeth in the aperture are represented by two categories that are designated as superfi cial and immerged. Superfi cial teeth are local thi cke- nings of the lip; immerged ones are natural result of narrowing of the last whorl at the fi nal stages of the shell formation. 2. Superfi cial teeth and lip serve basically for squeezing mucus from the mantle collar and the epiphragm formation. 3. Superfi cial teeth there are mostly in mollusks, that dig themselves into the soil to survive of dry season. 4. The main function of immerged teeth is providing an optimal orien- tation of shell with visceral sack with respect to the cephalopodium and improvement of management of high shell. 5. With increase of aridity, amplifi cation of the superfi cial teeth and weakening of immerged teeth is observed. This trend is clearly seen in the species that live openly and survive the drought by gluing to the substrate using the aperture. 6. Snails inhabiting steppe, desert and semi-desert areas often lack ape- rtural teeth and well developed lip. In such species, a wide lapel of the aperture is often developed, contributing to tight contact of the aperture to the substrate.

© A.A. Schileyko, 2016. 270 A.A. Schileyko

In several families persisting in the current geological time, there is a clear trend toward shell reduction leading to the formation of a slug morphotype, with all reduction stages being observable over their mem- bers. Reduction of shell stimulates perfecting of the nervous system and improvement of physiological features of mollusks, as well as growing complexity of their behavior. From this a forecast follows that this trend will be intensifi ed in the course of further evolution of Stylommatophora.

ФУНКЦИОНАЛЬНЫЕ ВЗАИМОСВЯЗИ КОНХОЛОГИЧЕСКИХ И АНАТОМИЧЕСКИХ ПРИЗНАКОВ У STYLOMMATOPHORA (MOLLUSCA, GASTROPODA) Анатолий A. Шилейко Институт проблем экологии и эволюции им. А.Н. Северцова РАН; [email protected]

Параметры раковины (форма, размеры) стебельчатоглазых моллю- сков (Stylommatophora) определяются историей развития группы, осо- бенностями их питания и условиями существования в разные геоло- гические периоды, а также эпигенетическими ограничениями. Кратко анализируется история развития стиломматофор начиная с палеозоя. Предполагается, что одним из важных факторов, опреде- ливших развитие стебельчатоглазых моллюсков, были особенности их пищевых объектов. Устье есть основной канал связи моллюска с внешним миром и любые элементы строения устья так или иначе связаны, прямо или опосредованно, с приспособлениями животных к условиям внешней среды. Поэтому особое внимание в статье уделено морфофункцио- нальному анализу устьевых структур. Основные результаты этого ана- лиза сводятся к шести базовым положениям. 1. Зубы в устье представлены двумя категориями, которые обо- значаются как поверхностные и погружённые. Поверхностные зубы представляют собой местные утолщения губы; погружённые часто появляются как естественный результат более или менее ярко выра- женного сужения последнего оборота на заключительных этапах он- тогенетического формирования раковины. 2. Поверхностные зубы и губа служат в основном для выжимания слизи из края мантии и формирования эпифрагмы. 3. Поверхностные зубы имеются большей частью у моллюсков, за- рывающихся в почву на сухой сезон. 4. Основная функция погружённых зубов — обеспечение опти- мальной ориентации раковины с висцеральным мешком по отноше- нию к цефалоподиуму и совершенствование управлением высокой раковиной. 5. С возрастанием аридности условий наблюдается усиление по- верхностных зубов и ослабление погружённых. Эта тенденция хоро- шо прослеживается у видов, живущих открыто и переживающих за- суху, приклеиваясь устьем к субстрату. Characters in Stylommatophora (Mollusca) 271

6. Улитки, обитающие в степных, полупустынных и пустынных местностях, часто лишены устьевых зубов и хорошо развитой губы. У таких видов часто развит широкий отворот края устья, способству- ющий плотному прилеганию устья к поверхности субстрата. В настоящее геологическое время в пределах нескольких семейств чётко прослеживается тенденция к редукции раковины, приводящая к становлению морфотипа слизней. В современной фауне прослежи- ваются все этапы утраты раковины. Редукция раковины стимулирует совершенствование нервного аппарата и физиологических характери- стик моллюсков, а также усложнение их поведения. Из этого следует прогноз, согласно которому в ходе дальнейшей эволюции стебельча- тоглазых эта тенденция будет усиливаться.

1. Introduction characters of molluscs should be considered Superorder Stylommatophora is a large fi rst of all, which look like adaptations to the (at least 25000 species) and diverse (about 90 conditions existing in arid zones. Morpho- families, 2600 genera and subgenera) group functional analysis often allows to advance of terrestrial pulmonate mollusks spread all a hypothesis on the adaptive signifi cance of over the continents (absent in the Antarc- this or that character. tic only, although present in sub-Antarctic The systematic study of terrestrial pul- islands). Although initially stylommato- monate mollusks constituting superorder phorans are moisture-loving animals, many Stylommatophora began, naturally, from of them have mastered almost all types of descriptions of their shells. Start of a broad biotopes and often form large aggregations. study of the internal organization of ter- Causes of prosperity (or, on the contrary, restrial pulmonate mollusks was laid in the oppressed state) of a group depend on 1855, when two fundamental publications external factors (climate, soil, topographic have been issued by a German priest Adolf features, geologic history of the region, hu- Schmidt and a French naturalist Alfred Mo- man activities) as well on the peculiarities of quin-Tandon. These studies have demonstrat- the organization of the animals themselves. ed the existence of a wide variety of char- Although the “peculiarities of the orga- acters, mainly those related to the structure nization” may include not only morphologi- of reproductive tract. Since that time, the cal, but also physiological, biochemical, and anatomical method began to be widely used ethological properties of organisms, here are in studies on the systematics and phylogeny considered predominantly morphological of pulmonate mollusks. characters, the adaptive signifi cance of which As a result, to date a vast array of infor- is the subject of the present discussion. mation about the internal structure of stylom- Main and permanent danger threatening matophorans has been accumulated, fi rst of land mollusks living in conditions of dry cli- all concerning reproductive tract and radula. mate (or in regions where the rainy periods However, the functional signifi cance of are regularly replaced by dry seasons) is the the structural features of these animals is danger of dehydration. Nevertheless, there rarely discussed. One of many examples of is a large number of species living and even ignoring the functional role of certain char- prospering in arid areas. Therefore, those acters is the division of Stylommatophora 272 A.A. Schileyko into four large taxa of ordinal rank according reproach their author in anthropomorphism. to the structure of the excretory apparatus However, any formulated and published hy- (Orthurethra, Sigmurethra, Mesurethra, and pothesis is a matter for further discussion, Heterurethra) proposed by Pilsbry (1900) during which it will be either confi rmed or and supplemented by Baker (1955). Actually, improved, or rejected and replaced by a more these four names describe, or characterize, correct interpretation of the known facts. In the four states of the excretory system, and any case, nomination of a hypothesis serves the most perfect condition (sigmurethria) can as an impetus to further analysis and/or al- occur independently in different evolutionary ternative interpretation of these facts and is branches of Stylommatophora (Schileyko, targeted to the obtaining of new ones. 1975, 1978a). Abbreviations used in the fi gures thro u- It should be noted that paleontologists, in ghout the article are as following: H – heart; contrast to zoologists, use very effectively the K – kidney; LL – left lobe of mantle collar; method of phylogenetic reconstructions based MC – mantle collar; P – pneumostome; PL – on the morphofunctional analysis of fossil pneumostomal lobe of mantle collar; R – rec- material from which they “squeeze” all what tum; RL – right lobe of mantle collar. is possible. One can point, as examples, to brilliant works by Bizikov (2008) on the evo- 2. Shell and soft body lution of cephalopod shells and by Parkhaev 2.2. Shell shape and size (2008) on minute Cambrian gastropods. Correct analysis of functional relation- 2.2.1. The shell shape ships between anatomical and conchological General analysis of the shell geometry features is impossible without taking into has been presented by Raup (Raup, 1966; consideration the biological aspects, espe- Raup, Stanley, 1971), who has selected four cially the behavior of snails during mating principal parameters: and feeding. 1. The shape of the generating curve. This It is impossible to discuss the problems curve is a line of crossing the widening tube of functional morphology without touching (cone) with the plane, in which lies the axis the problems of phylogeny, because one of of coiling; in the majority of gastropods it the most natural assumption of morphofunc- coincides with the plane of aperture. tional analysis is the assumption that some 2. The whorl expansion rate. The rate of taxon has originated from some other taxon. the widening of generating curve as it rotates Nor less closely the morphofunctional around the axis (= rate of the whorl widening analysis is related to the problems of sys- = step from the axis). This is a relation of any tematics. For example, the understanding of linear size (for example, of the diameter) of the nature of such a feature as the topography two generating curves separated by the entire of the retractor of right (in sinistral forms — whorl of the spire. left) ommatophore has permitted to suggest 3. The position of the generating curve that xerophilic Hygromiidae do not constitute in relation to the axis. This parameter is im- a separate taxon Helicellinae, but is a result portant in some cases when the generating of parallel evolution (Schileyko, 1978b). curve is not a circle. Obviously, the conclusions based on 4. The rate of whorl translation (= trans- morphofunctional analysis, are speculative lation of the whorl, = step along the axis). to some extent and often give the reason to It is expressed best of all by the relation of Characters in Stylommatophora (Mollusca) 273

Fig. 1. Shells with partly uncoiled lower whorls. Above is Tetrentodon barroi Ja- ume et Torre, 1972, below is Gyraxis brooksiana (Pfeiffer, 1859). After Schi- leyko (1998–2007). Рис. 1. Раковины с частично развёрнутыми последними оборотами. Вверху – Tetrentodon barroi Jaume et Torre, 1972, внизу – Gyraxis brooksiana (Pfei ffer, 1859). По Schileyko (1998–2007). shifting along to the axis at any part of the of shell usually allows a confi dent judging whorl around the axis. about the familial belonging of the species. These four parameters usually do not However, the shell, for example, of helicoid change during growth, although exceptions appearance is characteristic for a number are known. For example, in Tetrentodon and of families (Xanthonychidae, Epiphrag- Gyraxis (Urocoptidae), all whorls form a mophoridae, Bradybaenidae, Camaenidae, tight spiral, except for 2–3 last whorls, which Pleurodontidae, Helicidae, Hygromiidae), are not in contact with each other, forming a so it is impossible to reveal to which family kind of corkscrew (Fig. 1). belongs the species with such shell without In the context of our consideration, two prior knowledge of anatomy (before all, the parameters are most important: the second structure of reproductive tract) of the given one (step from the axis) and the fourth one species. (step along the axis). An increase the step A more or less swollen last whorl is char- from the axis leads to formation of fl attened acteristic for helicoid shells. This whorl is a shell, while that of the step along the axis container for withdrawal of cephalopodium: leads to the arising of elongated shell. the larger relative volume it (cephalopodium) The general habitus of the shell is mostly occupies, the more is the volume of a foot and determined genetically, and many taxa of the hence the more muscular effort it can develop family rank are characterized quite defi nite- and more effectively the animal can burrow ly by conchological characters. To such taxa into the soil for aestivation or hibernation. one could attribute, for example, Vertigini- At the same time, the presence of relatively dae, Gastrocoptidae, Pyramidulidae, Valloni- bulky cephalopodium means that the animal inae (Valloniidae), Speleodiscidae, Strobi- is able to store a suffi ciently large volume of lopsidae, majority of Enidae+Pachnodidae, water in its tissue, which is essential for the Megaspiridae, Urocoptidae, Clausiliidae, snails living in regions with a dry season. It Succineidae, where the external appearance is clear that much swollen last whorl is ob- 274 A.A. Schileyko

Fig. 2. Morphological series of Oleacinidae. Рис. 2. Морфологический ряд Oleacinidae. From left to right/Слева направо: Varicella, Guillarmodia, Plicoleacina, Strep- tostyla, Strebelia. served, as a rule, in more or less depressed hence, the shift of the pneumostome up- shells (from globular to fl at). In such shells, wards (i. e. towards the top) allows if not to there is often a peripheral angle or keel, the avoid completely this problem, but at least function of which will be discussed below. to minimize it substantially. The same rea- Concerning high-spired shells, i. e. those son explains the fact that the anterior end of having the height greater than the diameter, the cephalopodium in Oleacinidae is mostly such a shape is usually achieved either by strongly elongated, and this allows the snail increasing the step along the axis, or by in- to penetrate deep into the soil, leaving pneu- creasing the number of whorls. mostome above its surface. The tendency to Increasing of step of whorls along the ax- increase relative height of aperture can be is is not always associated with a signifi cant traced in the series of Varicella — Guillar- increase in the number of whorls, but such modia — Plicoleacina — Streptostyla — shells usually have relatively high aperture. Strebelia; if a number of other representa- The results of evolution in this way can be tives of oleacinids are included in this row, seen on the example of Oleacinidae (Fig. 2). one can make it even smoother. Now the question is, what such shells give Another feature associated with the ap- to their owners? As the oleacinids are preda- erture is its orientation relative to vertical tors, they, to search of a prey (mainly oligo- plane (i. e. the plane of the columella loca- chaetеs, soil larvae of some insects and other tion). Aperture can be straight, i. e. its plane snails), often must penetrate the soil holes. is parallel to the vertical plane, or can be lo- The depth of immersion of the molluscan cated at an angle to it. In the latter case, the head to the hole is mainly limited by the po- aperture is usually more or less descending sition of the breathing orifi ce. Evidently, if in front, i. e. at a standard position the upper the angular region, where the pneumostome margin of the aperture is closer to the observ- occurs, squeezes into a narrow passage, some er than the lower one. The stronger the last diffi culties with breathing may appear. The whorl descending, the more the plane of the formation of a relatively high aperture, and aperture deviates from the vertical plane. In Characters in Stylommatophora (Mollusca) 275

Fig. 3. Shells with aperture turned upward, after Schileyko (1998–2007). Рис. 3. Раковины с обращённым вверх устьем, по Schileyko (1998–2007). From left to right/Слева направо: Hypselostoma roebeleni (Moellendorff, 1890), Campolaemus perexilis (Smith, 1892), Boysia boysii (Pfeiffer, 1846), Anostoma depressum Lamarck, 1822.

fl at lenticular shells the angle of inclination the fact that defi nitive aperture, which often of the aperture can reach nearly 90º, i. e. in carries various structures (in particular, the this case the aperture is almost horizontal. In lip which promotes to tight adjoining of the very rare cases [Hypselostoma, Campolae- aperture to the substrate) that minimizes the mus, Boysia (Hypselostomatidae); Anostoma adverse external effects, in juveniles have (Bulimulidae)] the aperture is also almost not yet been formed, and the presence of ju- horizontal, but its plane turned upward, to venile keel partly compensates the absence the side of shell apex (Fig. 3). of “adult” structures. The reasons causing the appearance of It is more diffi cult to understand the rea- strongly descending aperture, are under- sons of appearance of the aperture, which standable from functional point of view: a plane is facing upward (like, for example, snail with such aperture, when sitting, say, in Anostoma). For the correct interpretation on the surface of a rock, can very tightly ad- of such a character, it is desirable to know join to the substrate and, is probably hardly the biology of the respective species, namely noticeable for some natural enemies (mainly how they survive the unfavorable seasons, amphibians, birds, and some mammalians). whether there is a difference in the conditions Besides, this orientation of the aperture cre- of life and behavior in young and adult in- ated a more effective resistance to such ef- dividuals, in particular their behavior during fects as, e. g., strong gusts of wind or water copulation, the peculiarities of their diet and fl ow in time of heavy shower. It should be movement. One of possible suggestions is added that strongly descended oblique aper- that such an aperture is associated with a ture is often associated with the formation of specifi c behavior during mating, since juve- an angle or even keel on the shell periphery, niles have a normal position of aperture, and which further increases its “streamlining”. turning of the plane of the aperture upward In this regard, an attention should be called begins just in subadult age, immediately be- to the fact that the angulated shell appears fore puberty. often in juvenile animals, whereas the angle Flattened shells (many Discidae, Trocho- may disappear in fully formed shells. This morphidae, Zonitidae and some ot hers) are phenomenon is appropriate to connect with often characteristic for the mollusks living 276 A.A. Schileyko

Fig. 4. Multi-whorled fl at shells. Аfter Schileyko (1998–2007). Рис. 4. Многооборотные плоские раковины. По Schileyko (1998–2007). From left to right/Слева направо: Cupulella valley Aguayo et Jaume, 1948, Hen- dersoniella palmeri (Dall, 1905), Coxia macgregori (Cox, 1870), Poly gyratia po- lygyrata (Born, 1778). under the bark of stumps, under stones and in as a rule, to the increased size of individuals. similar places. Reasons for such a correlation 3. Biology of the species, i. e. peculiarities are obvious. Increasing of the whorls number of nutrition, microhabitats, life cycles, and often also leads to formation of high shells breeding peculiarities. [fl at shells with a large number of whorls The fi rst and second of the above factors as Cupulella (Subulinidae), Hendersoniella do not require explanations. The third factor (Urocoptidae), Coxia (Trochomorphidae), deserves a more detailed consideration, be- and Polygyratia (Camaenidae) are extremely cause it is closely linked to the evolution of rare (Fig. 4)]. In such shells the number of terrestrial Pulmonata as a whole. This prob- whorls can reach 30 or even more, and they lem was discussed by the author (Schileyko, are characteristic for the majority of Enidae, 2014), so I repeat here in brief the principal Clausiliidae, Megaspiridae, Bulimulidae, points only. Urocoptidae. Snails with a similar elongated The most ancient fi ndings of terrestrial shell live mostly (though with many excep- pulmonate mollusks are dated by the Paleo- tions) on vertical surfaces — rocks, stems zoic (Carboniferous–Permian) (Solem, Yo- of grasses and shrubs, etc.). Freely hanging chelson, 1979) (Fig. 5). Noteworthy, all of shell requires minimal effort to control it them had a small size: the height of the bul- (more on this matter see below). let-shaped shells (Dendropupa, Anthracop- upa) do not exceed 8 mm (Dendropupa ve- 2.2.2. The shell size tusta), but usually they were even smaller, no The size of mollusks is determined by at more to 5 mm; the diameter of the fl attened least three factors: 1. Genetic nature of the ones (Protodiscus) is no more than 2.5 mm. species setting certain limits on their vari- Evidently, the very fi rst steps of pulmo- ability (for example, no one member of Ver- nates moving from the sea onto the land were tiginidae can reach the size of the smallest made through the littoral. At that time, the species of Achatinidae); 2. Environmental only source of their food was detritus — an conditions. This circumstance is often asso- organic material produced by the shallow- ciated with the intraspecifi c variation: in gen- water inhabitants including content of bac- eral, more favorable conditions of life lead, terial and algal mats. Hence, for the earliest Characters in Stylommatophora (Mollusca) 277

Real heyday of the land pulmonates has started since the early Cretaceous simulta- neously with widespread distribution of the angiosperm plants, which provided abundant and inexhaustible food resource. It should be underlined that the modern pulmonates do not eat either gymnosperms or ferns or mosses, which form the basis of the planet fl ora throughout most of the Mesozoic. Starting from the upper Cretaceous, there appeared members of several Recent fami- lies as well as a number of groups extinct subsequently (Fig. 6): Filholiidae (middle Eocene to lower Oligocene of Europe), Anadromidae (upper Cretaceous to middle Eocene of Europe and north-western Africa), Fig. 5. Paleozoic Stylommatophora. Аfter So- Grangerellidae (Paleocene of North Ameri- lem, Jochelson (1979). ca; though it is possible that they belonged Рис. 5. Палеозойские стебельчатоглазые. По to Bulimulidae, like Recent South American Solem, Jochelson (1979). Anostoma) (Zilch, 1959–1960). Among Cre- taceous and especially Tertiary pulmonates, non-marine mollusks the initial type of their there were, inter alia, shells of large size, nutrition was detritophagy and probably bac- for example, Hodopoeus (Lysinoeinae of teriophagy. If so, it is clear that with such Humboldtianidae, or perhaps Pleurodonti- a low-calorie food the mollusks could not dae) with shell diameter up to 60 mm (Eo- reach large size. cene or Paleocene of the southern USA), or

Fig. 6. Representatives of extinct families of terrestrial Pulmonata. Рис. 6. Представители вымерших семейств наземных Pulmonata. 278 A.A. Schileyko

Fig. 7. The largest Recent Stylommatophora. Аfter Schileyko (1998–2007). Рис. 7. Самые крупные современные стебельчатоглазые. По Schileyko (1998–2007). Achatina fulica Bowdich, 1822, Megalobulimus popelairianus (Nyst, 1845), Neobe- liscus calcarius (Born, 1780), Ryssota otaheitana ovum (Valenciennes, 1827), Bertia cambojiensis (Reeve, 1860). the above mentioned Filholiidae with shell 7. Polyphages. Eat everything, includ- height up to 100 mm. ing the rotting parts of the angiosperms and Consequently, since the Cretaceous the organics of animal origin. Dimensions not limits for dimensional characteristics of the limited. shell disappeared to an extent in which it Thus, the members of four groups of the (size) depended on the food peculiarities. above seven can reach a large size (Fig. 7). Recent stylommatophorans, according to The largest snails are African Achatinidae their diet, can be divided into the following (Achatina, shell height up to 200 mm), South seven groups: American Megalobulimidae (Megalobu- 1. Detritophages/bacteriophages. Mol- limus, height up to 160 mm), Subulinidae lusks of small size. (Neobeliscus, height up to 120 mm), Philip- 2. Micromycophages. Feed onfungihy- pinian Ryssotidae (Ryssota, diameter up to phae. Snails of small size. 120 mm) and south-eastern Asian Dyakiidae 3. Macromycophages. Eat fruit bodies (Bertia, diameter up to 80 mm). of macromycetes. Dimensions not limited The snails of larger size usually live on (slugs not considered). the earth surface, often bury into the soil 4. Lichenophages. Feed on lichens. Mol- during the unfavorable for an active life, or lusks of small or middle size. (in the tropics) sometimes climb the trees. 5. Phytophages. Feed on living tissues of The peculiarities of biology of the species angiosperms. Dimensions not limited. upon which the size of mollusks depends also 6. Predators. Feed on other mollusks (in- on the nature of preferred habitats. So, snails cluding cannibalism), soil oligochaete worms inhabiting forest litter have, as a rule, small and insect larvae with soft coat. Dimensions or tiny size (typical examples are Vertigo, not limited. Punctum, Euconulus, Pristiloma, Hawaiia). Characters in Stylommatophora (Mollusca) 279

Fig. 8. Examples of complex columellae in Urocoptidae. Аfter Schileyko (1998–2007). Рис. 8. Примеры сложных столбиков у Urocoptidae. По Schileyko (1998–2007). From left to right/Слева направо: Amphistemma pilsbryana (Ramsden, 1914), Idio- stemma uncata (Pfeiffer, 1859), Ezochara strangulata (Pfeiffer, 1856).

2.3. Aperture 2.3.1. Juvenile characters The aperture is the main channel of com- One can distinguish three types of aper- munication of the mollusks with the outside tural structures according to the time of their world, so any elements of the apertural struc- formation in the ontogenesis: ture are related, directly or indirectly, with 1. Those appearing in embryogenesis or the adaptation of animals to any external fac- early postembryogenesis and in the process tors. Therefore it is natural that the main em- of the shell growth but not undergoing sig- phasis here is given to the functional analysis nifi cant qualitative changes (for example, of peculiarities of the apertural structures. columellar plates in Orculidae, Urocoptidae, In a number of terrestrial pulmonates, ju- Clausiliidae and some others). venile shells differ from fully formed ones 2. Shell grows, without having the aper- mainly by a number of whorls and, conse- ture armature, until reaching subadult state quently, by their size (Achatinidae, many when the fi nal portion of the shell is com- Subulinidae, Charopidae, Zonitidae, most pleted quickly together with the elements of of Helicarionidae, some Hygromiidae, Suc- armament of the aperture (numerous Pupilli- cineidae, etc.). More often, however, the fully dae, Gastrocoptidae, Vertiginidae; a number mature individuals develop adult characters of Bulimulidae; some Helicidae (Ariantinae), which testify puberty and cessation of growth for example, Causa, Isognomostoma; some of the mollusk (narrowing of the last whorl, Hygromiidae, for example, Chilanodon, ma- the presence of lip and aperture armament). ny Polygyridae and some others). Such features are usually called defi nitive 3. During the shell growth the elements characters. of apertural armature are periodically formed In the adaptive and evolutionary mean- but they are not related to the defi nitive ap- ings the juvenile and defi nitive characters erture. Such elements may be called “pseu- are equivalent and clear boundaries between dodefi nitive” (palatal and/or basal folds in them may be absent. But each of the age Lauria of Orculidae; Plectopylidae, some groups has its own, sometimes quite sub- Streptaxidae, Clappiella of Zonitidae, Paed- stantial, functional peculiarities, so there is hoplita of Hygromiidae, the lips in many a reason to consider them separately. Hygromiidae). 280 A.A. Schileyko

Fig. 9. Postembryonic development of Sphyradium doliolum shell. Aper tu- ral teeth in black. After Schileyko (1984). Рис. 9. Постэмбриональное развитие раковины Sphyradium doliolum. Ус тьевые зубы зачернены. По Schileyko (1984).

The fi rst type is associated with the his- shell growth, or periodically with the shell tory of the taxon, often has atavistic nature, growth: examples are Triangustoma, Gemi- and not in all cases amenable to functional nula, Chondrulopsina (Enidae), Paedhoplita analysis. Sometimes columellar plates exhib- (Hygromiidae), Isognomostoma (Helicidae), it a great variety and can reach a consider- many Polygyridae. able complexity (the most striking example The second and third types are usually as- is Urocoptidae, Fig. 8), and in such cases sociated with the peculiarities of the biology they deserve a special consideration from and behavior of snails at different stages of the functional point of view. Unfortunately, their postembryogenesis (see below). I do not have enough material for such an These types of apertural structures are analysis. At the moment I can only assume convenient to consider on the example of the that such a complex structure linked to the three species in Orculidae: Sphyradium dol- thin differentiation of columellar muscle that iolum (Bruguière, 1792), Pagodulina lederi facilitates the management of a high shell (O. Boettger, 1886), and Lauria cylindracea inherent to urocoptids. (Da Costa, 1778) (Schileyko, 1967а, 1984). As regards barriers located on the parietal The following information is based on direct wall of the aperture, they, in dependence on fi eld observations, though not supported by their origin, can be attributed either to the strictly quantitative data. category of immerged or superfi cial. The juveniles of S. doliolum live under So, the long parietal plate that appears conditions that are signifi cantly different at the end of embryogenesis, is homologous from those of the adults. These snails repro- to columellar plate and belongs to the cate- duce by laying 10–20 eggs in various shel- gory of immerged. The point is that there is ters. Shell of newly hatched snails consists no clear boundary between parietal and col- of one and a half whorls (Fig. 9), bearing umellar margins of the aperture and parietal spiral striation. These shells have already two plate is the columellar one that shifted to apertural structures: tubercular parietal and the parietal wall. Parietal plate of this type thread-like columellar plates. By forming the occurs, for example, in some Orculidae. Pa- fi rst postembryonic whorl, the length of the rietal teeth of superfi cial type are not con- parietal plate is about 3/4 of its length. By nected with columella and can be formed 2–3 shell whorls, this plate occupies almost either immediately before completion of the entire last whorl. When the shell has 3–4 Characters in Stylommatophora (Mollusca) 281

snails live already alongside with adults and but they need intensifi cation of protection of the aperture because the adult characters are not yet developed. In addition, at this time the snails eat particularly intensively, fi lled crop takes a signifi cant volume, and parietal plate, serving as a partition between the crop and rectum, is a limiter which min- imizes pressure on the rectum from the crop. Accordingly, just at this time parietal plate Fig. 10. Position of apertural teeth in Pagodu- reaches the strongest degree of its develop- lina lederi. ment. More information about the role of Рис. 10. Положение устьевых зубов у Pago- the parietal plate can be found below in the du lina lederi. section “defi nitive characters”. Finally, the mature animals are associated defi nitive whorls, the appearance of the pa- mostly with the middle and especially with rietal plate changes: its lower edge becomes the upper layers of litter or rock slide, already undulating. The plate reaches its maximum having adult features: the aperture with lip development at the stage of 5–8 defi nitive and narrowed last whorl. whorls. With the growth of the last whorl, The shell of P. lederi that lives mainly in the wave-like undulation of the plate edge the litter of deciduous forests basically has disappears. At the early stages (1–4 whorls) same features of postembryonic morphogen- the length of the plate is not more than one esis as in S. doliolum, but, speaking fi gura- whorl; at 5–6 whorls the parietal plate length tively, in a synoptic form. can reach two whorls, with further growth Young pagodulinas, just hatched in the its length decreases again. Finally, in the lower layers of the tree waste, lack any teeth fully formed shells the parietal plate usual- in the aperture until they reach the subadult ly extends up to 1–1.2 whorls (sometimes a age (at 6–7.5 whorls) (Fig. 10). At this stage, little more). a rather powerful aperture armament has Taking the above into consideration, let formed: columellar and parietal lamellae us consider the conditions under which the and an elongate-tubercular palatal fold. At snails live at early stages of their postem- this time, these mollusks which have not yet bryogenesis. Since the eggs are deposited reached puberty already live together with in secluded and strongly shaded areas, the adults, but do not have the adult structure, newly hatched snails turn out in the same the lip. Since the lip is absent, but the need conditions. With further growth the young to protect the aperture does exist, at this stage snails crawl up to the middle and upper lay- the provisional structures has formed, which ers of the forest debris or litter, since for pu- perform, probably, the function of protection. berty these thermophilic animals require a Upon reaching full number of whorls (8–8.5) certain minimal insolation. Aperture of the and with the formation of a lip, above men- snail at that time is still relatively wide, and tioned teeth appear deep inside the last whorl due to the growth of the shell, the role of the and over the time undergo some reduction. parietal plate as a means of protecting the Thus, a provisional apparatus formed en- aperture increases. At subdefi nitive age the tirely at subadult stage of the shell formation 282 A.A. Schileyko

Fig. 11. Provisory basal plicae in Lauria cylindracea translucing through the shell wall at various stages of postembryogenesis. Numerals show the number of whorls. Figures are reduced to the same size. After Schileyko, 1984. Рис. 11. Провизорные базальные складки у Lauria cylindracea, просвечивающие сквозь стенку раковины на разных стадиях постэмбриогенеза. Цифры по- казывают число оборотов. Изображения приведены к одному размеру. По Schileyko, 1984. and, having fulfi lled its function, becomes ly, and at reaching of the full number (6–7 resorbed. whorls) they are either completely absent In L. cylindracea, unlike the previous two (most often), or one of them is present at species, provisional structures arise sever- the beginning of the last whorl (eventually, al times during the shell growth (Fig. 11). totally resorbed). It is an ovoviviparous species, with each Parietal lamella in L. cylindracea appears, individual producing from 4 to 12 young as mentioned above, in late embryogenesis snails. The shell of a newly born snail has and lengthens with molluscan growth. In about two whorls and is equipped with one the course of further development the upper weakly developed parietal lamella. When the part of the plate disappears, remaining only a fi rst postembryonic whorl is formed, there trace in form of a thin, light strip. Lamella as appears a radial fold on the basal edge of such is always present only in the last whorl, aperture, and at the stage of 3.5 whorls the having almost constant height; when lip is number of folds achieves usually three. At formed, the lower end of the plate thickens, the stage of 4 whorls the resorption of the turning into tubercular tooth. fi rst, i.e. posterior fold begins. Based on the above description of the The maximal number of simultaneous- morphological transformations, let us con- ly present basal folds does not exceed 5, sider some aspects of life of a juvenile snail and such a situation is observed until 4.5–5 after its birth. whorls of the shell. With further growth, the Before the birth of young snails, the pa- new basal folds begin to appear less regular- rental individual descends into lower layers Characters in Stylommatophora (Mollusca) 283

considered above (Sphyradium, Pagodulina and Lauria) it is characteristic that maximal development of provisional structures is reached at that stage of postembryogenesis when the juveniles occupy the same spatial niche as the mature animals, but have not yet the “adult” protecting structures. The third type of apertural structures formed in postembryogenesis (periodic ap- pearance of lips and provisory teeth) is not so rare. In addition to the mentioned species of the genera Clappiella and Paedhoplita, this phenomenon is observed, for example, in a number of species of Varicella (Oleacin- idae), some Streptaxidae, Aegopis verticillus Fig. 12. Parmacella ibera. A – juvenile shell. (Lamarck, 1822) (Zonitidae), Kokotschash- B – region of juvenile shell, enlarged. C – vilia phaeolaema (O. Boettger, 1886) (Hy- “operculum”. D – shell of adult specimen. gromiidae). In these cases, the young snails Рис. 12. Parmacella ibera. A – ювенильная ра- live together with the adults and are equally ко вина. B – ангулярная область ювени ль- exposed to the same unfavorable life con- ной раковины, увеличено. C – «крышечка». ditions. D – раковина взрослой особи. It is appropriate to mention a very in- teresting way of survival of the dry season, of debris or litter (possibly newly born snails which is observed in some juvenile slugs, in crawl downward). Consequently, young in- particular, in Parmacella ibera Eichwald, dividuals fi nd themselves in the conditions 1841 (Schileyko, 1967b). Here we see a com- of constant and almost complete shade and bination of morphological and biological pe- high humidity. In such a situation, the weak culiarities of the species. Under conditions of development of aperture armament looks the south-eastern Transcaucasia, the hatch- as a natural phenomenon. The growing ing takes place in late June to early July, i. snail still leads a secretive mode of life, but e. at the peak of the dry season. Just hatched gradually rises to higher layers of the litter, mollusk has a shell, consisting of approxi- where the problem of the water balance is mately one and a half whorls, where it can somewhat exacerbated but yet not leading. entirely hide. This embryonic shell (nucle- In this regard, the appearance of basal folds us) is provided with an “operculum” which is observed; these folds are functionally anal- is seemingly a modifi ed epiphragm. In sub- ogous to the defi nitive lip. Upon reaching angular region of the aperture there is a tiny of 4 whorls the snail lives already together horseshoe-like plica, both ends of which are with adult individuals, but needs additional directed outward (Fig. 12). Over the space protective devices, since its lumen of aper- between the branches of this plica, there is a ture is relatively larger than in adults, and beak-like outgrowth of the aperture margin. the highest number of basal folds, as well as When the “operculum” is closed, the the degree of their development reach the notch on its upper left margin corresponds maximum at this time. For all three cases to the lower branch of horseshoe fold; the 284 A.A. Schileyko stalk of the “operculum” which is placed 2.3.2.1. Superfi cial teeth and over the notch enters the space between the adaptations to arid conditions upper branch of the horseshoe and the inner The lip is formed because at subdefi nitive surface of the suture. In this place, the stalk state, at the completion of the shell growth, grows together with the upper branch of the the mantle margin (mantle collar) rapidly fold and, at a shorter distance with palatal secretes calcium carbonate that is deposited margin of the aperture which is slightly on the entire free edge of the aperture in the bent down. form of a radial thickening, which is the lip. When the “operculum” is opened from When there are zones of especially intensive inside under the pressure of the animal’ body, secretion in the mantle collar, the local thick- the stalk is bent and the “operculum” rests enings of the lip are generated, which are su- against the beak-shaped outgrowth of the ap- perfi cial teeth in such places (Edlauer, 1941). erture, which limits the degree of opening of One of the most important factors of life the “operculum”. When the animal hides in a of the land snails is the water regime. As stat- shell, the stalk, by virtue of its elasticity, re- ed above, the main, and often the constant turns to its original state, tightly shutting the danger is the risk of dehydration. Water loss aperture. In time of coming the wet season performs mainly through the aperture. Obvi- the “operculum” falls off (because the stalk ously, improving the mechanisms that slow dries out, loses its elasticity and breaks down down the evaporation is, perhaps, the most under the pressure of the snail body) and the urgent problem, which strongly depends on horseshoe-like plica resorbs. the habitat conditions. There are three methods to slow down the 2.3.2. Adult characters evaporation rate: (1) the narrowing of the last In most cases, the adult (defi nitive) char- whorl toward the aperture; (2) the formation acters are related, directly or indirectly, to of a lip and superfi cial teeth; (3) the forma- the formation of apertural armature. The full tion of a protective fi lm (epiphragm) due to set of aperture armature includes a lip hav- the intensifi cation of mucus secretion by the ing the form of radial folds, and a system of mantle collar. The third method usually sup- teeth of various size and shape, located in- plements the fi rst two. side the aperture. Topographically, the teeth The snails usually survive dry seasons by are of two types, designated as superfi cial one of two ways.The nature of morphological and immersed (Schileyko, 1984). Superfi cial adaptations in representatives of these two teeth are arranged in the plane of the aperture ecological groups is considerably different. margins and represent local thickening of the 1. Mollusks digging into the soil orhiding lip. Immersed teeth are mostly not connect- deep into crevices of rocks [for example, Val- ed with the lip and located in the aperture at lonia, Oligolimax; Cahillus greggi (Miller, some distance from its margin. 1972) lives even in the extremal conditions The differences between the superfi cial of Death Valley (Johnson Canyon)]. These and the immersed elements of the aperture snails devoid of these features, but in reality armament are very serious not only from they exist in the low horizons of rockslides the topographic but also from the functional or deeply in the cracks of the rocks, where point of view, that is why these categories the temperature is lower and humidity is al- deserve separate consideration. ways high enough. Characters in Stylommatophora (Mollusca) 285

2. Mollusks live openly, glued with the intimescens (Martens, 1874), bears only mucus to a stones, dry leafs or other fl at sur- slight swelling, whereas in closely related faces. Such species often form aggregations species Ch. fedtschenkoi (Ancey, 1886), (clusters) on grass or bush branches (for ex- these swellings are transformed into strong ample, some Pachnodidae, a number of Bu- tubercles, with the degree of their develop- limulidae, Sphincterochilidae and majority ment varying, however, greatly depending of xerophilic Hygromiidae, like Xeropicta or on the conditions under which the given Xerolenta). They are characterized by three population lives. It should be clarifi ed that features: a toothless mouth, white shell color the mucus, which is used for formation of (usually with dark bands or radial stripes), the epiphragm, is actually taken not from as well as by a behavioral feature — they cephalopodium, but from the mantle collar, often form dense aggregations (clusters). since at the snail withdrawing into the shell, All three features can be regarded as adapta- the cephalopodium does not contact with the tions to life in arid conditions: lack of aper- aperture margin, and even more so with the tural teeth provides the fast retraction of the lip. This can be seen if to fi x the shell by a snail into the shell; white shell refl ects the piece of plasticine, orienting it by the aper- sun rays, reducing the risk of overheating; ture toward the observer, and to direct to it a temperature within the cluster lower than strong ray of light from an illuminator.In this the temperature of the ambient air (Fischer, case it is easy to notice that when the snail, 1971; Yom-Tov, 1971; Schmidt-Nielsen et al., irritated by light and heat fl ow is drawn into 1972; Boss, 1974). the shell, the inner margins of the aperture First, let us consider adaptations that are together with lip and teeth are immersed in characteristic for the mollusks which bury the thick of mantle collar, and the mucus themselves into the soil to survive the dry from the cephalopodium is wiped off not by season. The close connection between the teeth (with which cephalopodium, I am re- conditions of humidity and the degree of peating, has no contact), but by the lobes of development of armament of the aperture in mantle collar (Fig. 13). Chondrula tridens (Müller, 1774) (Enidae) Only when cephalopodium is fully with- has been shown by Matyokin (1950). Ac- drawn, and the central hole of the mantle cording to this author, the drier are the condi- collar, where the cephalopodium was retract- tions, the more intensive is the development ed, is covered by a thin fi lm of transparent of the apertural teeth. Thus, Matyokin has mucus, the withdrawing of the mantle col- concluded that the teeth serve for squeez- lar begins. Just at this moment the mucus is ing of mucus from cephalopodium during removed from the surface of the mantle col- withdrawal of the snail into the shell; the lar, using lip and teeth. The mucus is satu- epiphragm is formed at the expense of this rated with granules of calcium carbonate, mucus. It is essential that in the mentioned and the epiphragm is formed due to just this species the elements of aperture armament mucus. Only the most central portion of the are classifi ed as superfi cial. Such a correla- epiphragm includes that mucus, which had tion occurs in other species that have su- previously been removed by the lobes of the perfi cial teeth, and not only at the popula- mantle collar from the surface of the cepha- tion level, but also at the interspecifi c. For lopodium (Schileyko, 1978). example, strongly developed lip of another I think, it is hard to disagree with the fact representative of Enidae, Chondrulopsina that in arid conditions an epiphragm satu- 286 A.A. Schileyko

Fig. 13. Mantle collar in Stylommatophora. Рис. 13. Мантийный валик у Stylommatophora. A – Sphincterochila candidissima (Draparnaud, 1801); B – Cernuella cespitum (Draparnaud, 1801); C – Caucasigena abchasica (Lindholm, 1927); D – Arianta arbustorum (Müller, 1774); E – Fruticocampylaea narzanensis (Krynicki, 1836); F – Lindholmiola corcyrensis (Férussac, 1839); G – Helicodonta obvoluta (Müller, 1774); H – Metafruticicola pellita (Férussac, 1832); I – Helix pomatia Linnaeus, 1758; J – Euomphalia strigella (Draparnaud, 1801); K – Theba pisana (Müller, 1774); L – Ponsadenia semenovi (Martens, 1864); M – Angiomphalia caele- stimontana (Tzvetkov, 1940); N – Leucozonella mesoleuca (Martens, 1882); O – Xeropicta derbentina (Krynicki, 1836); P – Lindholmomneme nordenskioldi (Martens, 1876); R – Cernu- ella neglecta (Draparnaud, 1805); S – Karaftohelix middendorffi (Gerstfeldt, 1859). Characters in Stylommatophora (Mollusca) 287 rated with granules of CaCO3 more effec- as the lip (immerged teeth may be present). tively retards evaporation than epiphragm, Therefore the relationship between degree containing just a little of calcium carbonate. of development of the elements of the aper- Consequently, one can assume that the more ture armament, properties of epiphragm, and moisture defi cit occurs in the given biotope, parameters of the water regime, which was the more stringent are requirements for the discussed above, seems to be typical for the properties of epiphragm and more intensely burying forms only. the zones of mucus secretion are working. As How, then, to minimize the water loss a result, two independent phenomena arise: in the species living openly, i.e. under the strengthening of the superfi cial armament most arid conditions? With onset of the dry and intensifi cation of saturation of epiphragm season, adult or subadult snails rise up on by calcium carbonate. Since the mucus from stems of grass or branches of bushes. While the foot surface is removed by the lobes of the animal crawls, it keeps itself on a vertical the mantle collar, the middle part of epi- surface due to sticky mucus secreted by the phragm is built due to this mucus whereas pedal gland located in the thickness of the the peripheral part is formed at the expense foot and opening by a horizontal slit beneath of the secret released by the mantle collar, the mouth opening. Then the snail stops and i.e. due to the same source as the superfi cial starts to withdraw itself into the shell, with teeth (Campion, 1961). its basal margin of aperture directed up- Thus, it is reasonable to assume that the ward, while the shell apex downward (this degree of development of apertural teeth is true for the snails with an elongated shell, really depends on parameters of the water in particular, for some Enidae). When the balance at the given place, but this rela- head and anterior part of the cephalopodium tionship is not direct, but “coupled” with have withdrawn, the basal margin of aperture the presence in the mantle collar the zones comes into contact with the substrate, and with intense secretion of mucus that is rich at this point the aperture stick by the mucus with CaCO3. This mucus is needed primar- secreted by the mantle collar. Now, when ily to enhance the moisture loss resistance the aperture is fi xed on the vertical surface, of the epiphragm; but the existence of the the rest parts of the cephalopodium retract mentioned zones determines formation of gradually; as withdrawing the palatal and the teeth on the aperture margins, contrib- columellar margins are lowering onto the uting in turn, squeezing of the mucus from substrate; parietal margin contacts with the the mantle collar. In other words, a side ef- surface in the last turn. As a result, at the fect of the development of the zones of en- end of retraction of the snail, the aperture hanced secretion acquired an independent becomes glued along its entire perimeter — adaptive value. partly by the mucus, secreted by the mantle In consideration of the snails attaching to margin, partly by the mucus which has been the substrate by aperture in the dry season, removed from the foot by the lobes of the as different from those burying in the soil, mantle collar. Description of these process- it is convenient to start with pronounced es is based on the direct observation of the xerophilic mollusks living openly. Very sig- withdrawal of representatives of the Enidae, nifi cant is the fact that the vast majority of Crymean Brephulopsis cylindrica (Menke, species belonging to this category lack not 1828), and Transcaucasian Georginapaeus only the teeth of superfi cial type, but as well hohenackeri (Pfeiffer, 1848), and Hygromii- 288 A.A. Schileyko dae — Central Asian Xeropicta candaharica to the axis of withdrawal and at longitudi- (Pfeiffer, 1846). nal arrangement of folds the mantle slips on them, like on rails. Therefore, the main role 2.3.2.2. Immerged teeth of immerged folds is different — namely, to Under the term “immerged teeth” I mean ensure the effi ciency of gas exchange, protec- primarily the elements of the palatal margin, tion of rectum and management of shell. This since the long, elongated along whorls pa- statement deserves a detailed consideration. rietal and columellar lamellae are not con- Aperture teeth, breathing, and excre- nected with defi nitive (adult) aperture, and tion. If a shell has parietal teeth, there is usu- have other origin and other functions. ally also a small tooth in the upper part of the Teeth located on the parietal and columel- palatal margin.The crests of these teeth are lar margins of the defi nitive aperture, are not directed towards one another.Together, these always connected with initial parietal and teeth form a kind of sinulus, which houses columellar lamellae. They can be formed pneumostome, anus and the opening of the independently and often belong to the cat- secondary ureter. Thus, the named teeth egory of superfi cial. protect these organs (chiefl y pneumostome) Sometimes the last whorl (in the high from the pressure of adjacent organs. This shells) narrows toward the aperture. In such is important, because in time of intensive cases, the perimeter of the subdefi nitive ap- feeding, the crop that adjoins to mentioned erture is reduced, but the perimeter of the organs, strongly swells and presses on them. mantle edge remains the same. As a result, It is clear that, at strong expansion of the the mantle edge crumples and the immersed crop, the rectum, secondary ureter and pneu- teeth have formed in the emerged indenta- mostome are so strongly pressed against the tions. This has been shown for Clausiliidae outer wall of the shell that certain problems (Edlauer, 1941) and is true, probably, for can arise with defecation, respiration and many other groups with “toothed” aperture. removal of excreta. This problem is particu- A pertinent question concerns the correla- larly sharp when the last whorl is narrowed. tions of emergence of the immerged palatal The presence of rigid partition between the folds and narrowing of the last whorl. Obvi- crop and pneumostome region eliminates ously, this is common and natural, but not (or, at least, attenuates) this problem and the obligatory. For example, Pseudonapaeus parietal tooth participates in such partition. otostomus (Westerlund, 1898) (Enidae) has a If we take into account that the main spindle-shaped shell, but the immersed teeth pressure is made from one side, it becomes in this species are absent; on the contrary, a clear why the parietal tooth is more “neces- representative of the same genus, P. entop- sary” than palatal; this explains the fact that tyx (Lindholm, 1925), has a tower-shaped the parietal tooth only (without palatal) is shell, but it has a longitudinal palatal fold observed much more often than palatal only inside the aperture. Immersed teeth may play (without parietal). a role in reducing evaporation (by reducing Immersed teeth and management by the area of the aperture), but this function is shell. Shell and visceral sack with lung cav- not primary. An elongated structure due to its ity can be likened to two cones inserted into shape is hardly effective for squeezing mu- one another. If the surface of these cones cus from the mantle collar, since this purpose is smooth, the inner cone can be rotated requires a structure, oriented perpendicular relative to the outer and vice versa. But any Characters in Stylommatophora (Mollusca) 289 rigid structure protruding into the cavity of The assumption by Paul seems fair: when the aperture, will inevitably play the role of an animal is crawling, the extended vertical a stopper that limits an ability to rotate the blade of columellar plate lies on the dorsal visceral sac inside the shell, and the system side of the cephalopodium, greatly limiting of apertural teeth creates very effective cou- the possibility of deviation of the shell aside. pling of shell with the body of the mollusk. Paul, however, overlooked that the same The parallel with two cones in this case is role is more effectively performed by the very conditional, because the visceral sack of saddle-like system formed by parietal and a snail is curled into a spiral. Torque in this subparietal plates interconnected by trans- case can occur only in the region adjacent verse thickenings: when a snail is crawling, to the aperture, i. e. just where the complex this “saddle” covers the foot dorsal side from of immerged teeth is located. Facilitating above and fi xes the shell in certain position in the management by the shell, the immersed relation to the cephalopodium. Such a rigid teeth slow down the speed of retraction: it is system is effective if the aim is to fi x the shell possible that just with this fact it is linked to in certain position relative to the foot with the absence of immersed teeth in a number the minimal effort of the columellar muscle. of small snails, living in condition of strong However, this system greatly limits the mo- temperature fl uctuations. A palliative way out bility of the shell thus making diffi cult its is observed, for example, in Truncatellina: a management. Species of Cochlicopa that lost small palatal tooth, located deep inside the the apertural armature are more progressive shell, increases the coupling between the in this regard, as they can actively manage body of the animal and shell without im- the shell. Both Azeca and Cochlicopa live peding at the same time the rapid retraction. mainly among forest litter, small twigs, tan- Similar situation takes place in some Enidae: gled grass, and their ability to manage the Pseudonapaeus entoptyx (Lindholm, 1925), shell actively under such conditions is, to my Siraphoroides moltschanovi (Likharev et mind, a valuable acquisition. Additionally, Rammelmeyer, 1952) (Schileyko, 1984). this is one of important circumstances that In this connection a rare modifi cation explain why Azeca has a relatively narrow of columellar plate should be noted when it area of a relic type and lives in a limited set raises parallel to the columella, expanding at of habitats, while species of Cochlicopa are the same time into a blade. Such a structure common almost everywhere in Holarctic. of the columellar plate is observed in Azeca Maintaining of shell in certain position is (Cochlicopidae), Vertilla (Vertiginidae), a a special case of its control, and an important few Euxinolauria (Orculidae) and in some role of immersed teeth often consists of keep- Achatinellidae. Comparing position of the ing the shell in more or less constant orien- shell in respect to the foot in crawling Azeca tation relative to the cephalopodium. In this and Cochlicopa, Paul (1974) drew attention regard shells of some Hypselostomatidae are to the fact that, in Azeca, the shell is oriented remarkable: their aperture is sharply lifted, parallel to the foot, while in Cochlicopa (in and its plane is almost horizontal .Therefore, which the columellar plate is absent) the shell when the animal is crawling, the summit of is sideward deviated up to 20–30°. the shell is directed downwards and prob- Paul connects this difference to the fact ably does not (or nearly does not) touch the that Azeca has the columellar plate of the de- substrate, i. e. the shell is a weigh. One can scribed structure while Cochlicopa has not. assume that, as a result, the problem of the 290 A.A. Schileyko most advantageous position of the visceral columellar plate is an archaic character; the sack with respect to the cephalopodium is ex- presence of this plate facilitates the manage- acerbated. So, there are four strong teeth with ment of the shell, and the latter is performed blunt tops in the aperture of the Hypselosto- more successfully with the tighter contacts ma species; I think that, leaning on the dor- of the columellar muscle with the plate. In solateral surfaces of the cephalopodium from its turn, the tight adjoining of the columellar right and left these teeth restrict the possibil- muscle is largely ensured by narrowing of ity of free movement of the shell and keeping the last whorl. Thus, in the groups targeted it in the most favorable position. In parallel, to increasing of the shell height, process of the combination of parietal and upper palatal immerged armature formation is observed, teeth creates a sinulus, i.e. these teeth fulfi ll some elements of which became more and also the traditional function of protection of more fi nely adapted to optimize the spatial pneumostome and rectum. orientation of the shell of moving animal. As noted above, the appearance of the Further development of the apertural ar- teeth of immersed type is often associated mament of immersed type in this direction with the narrowing of the last whorl, which in a number of groups has reached an evolu- leads to the formation of fusiform shell.The tionary dead end: a complex system of teeth role of this process and evolutionary con- blocked up the aperture to such a degree that sequences deserve a special consideration. the rate of withdrawal of the snail into the Narrowing of the last whorl is prob- shell decreased signifi cantly, and the amplifi - ably an ancient acquisition that repeatedly cation of the drought became fatal, especially occurred during evolution of the stylom- for the animals with small size. matophoran mollusks; indeed, species with Snails with strongly toothed aperture fusiform shells are common among Chon- spend more time than animals of same size drinidae, Enidae, Bulimulidae, Urocoptidae, and with same whorls number but with a Clausiliidae, and Streptaxidae. Therefore simple aperture. the assumption that this narrowing had oc- This difference is related to the fact, that curred originally without any connection to if the aperture is devoid of teeth, just a sim- some external conditions, but was acquired ple contraction of the columellar muscle is subsequently to facilitate the management of enough for withdrawing the snail. If there elongated shell, or for keeping it in the most are numerous hard projections in the lumen favorable position relative to cephalopodium, of the aperture, then withdrawal, in addition looks quite plausible. to retraction of the axial musculature, needs Reducing of the diameter of the last whorl a complex redistribution of hemolymph in provides a closer mutual adjoining of the the body sinuses, which requires additional internal organs, and, in particular, a more time. tight contact of the columellar muscle with The way out of this evolutionary dead end the columella. Narrowing of the last whorl was found by snails, which had not time to may be compared with taut corset: a gym- reach an irreversible and critical specializa- nast or a dancer, with his clear and strictly tion of complex aperture apparatus.It can be coordinated movements, tightly tightens the assumed that their development has gone to- belt that plays the role of an outer support ward a fi ne functional differentiation of the during body turnings (Schileyko, 1984; Su- columellar muscle to maintain their ability vorov, 1993). It was shown above that long for a quick withdrawal. Characters in Stylommatophora (Mollusca) 291

This hypothesis is based on the follow- ravines, on the surface of the wet rocks, etc. ing facts. [E. rectidentata Schileyko, 1975, E. pauli- 1. Among Vertiginidae and Gastrocopti- nae (Lindholm, 1913)], have immersed ele- dae, the species with the strongest aperture ments of aperture armament only. E. pulchra armament were found in representatives hav- (Retowski, 1883) lives in the driest places: ing become extinct by middle of the Neogene this species inhabits deciduous forests in the (Steklov, 1967; Steklov, Tzytovich, 1967). northwest Caucasus, which dry out especial- 2. The most archaic and weakly variable ly strongly. And it is this species that devel- species among Recent Vertiginidae is Ver- oped, in addition to the immerged teeth, the tilla angustior (Jeffreys, 1830) which has a superfi cial tubercles and folds playing a role complex aperture. On the contrary, the most in protecting the body of the mollusk from evolutionarily advanced species, with a huge the moisture loss. distributional area, a very wide amplitude On this conformity to natural laws the of variability and often abundant, Vertigo peculiarities of ecology of individual spe- modesta (Say, 1824) shows a tendency to cies may superimposed. So, judging from the reduction of teeth down to their complete structure of the aperture E. tenuimarginata disappearance. (Pilsbry, 1922) is the most moisture-prefer- 3. Members of the genus Euxinolauria ring species. It lacks the superfi cial teeth, (Lauriinae, Orculidae), mostly inhabiting but has a wide lapel of the aperture margin western regions of the Caucasian Isthmus, that provides a snug fi t to the substrate. E. live only in forests, especially in relic for- sinangula Schileyko, 1975, in contrast, has ests, and are characterized by complex ar- not only immersed, but also superfi cial teeth, mature of the aperture. The representative of this fact suggesting the greater resistance to monotypic (perhaps, oligotypic) genus Lau- drying out than E. tenuimarginata. E. super- ria cylindracea (Da Costa, 1778), with vast structa (Mousson, 1876) occupies an inter- area and wide spectrum of biotopes, from mediate position. sparse deciduous forests to semi-deserts, has Quite a different situation exists in the a relatively simple aperture equipped with a conditions where Lauria cylindracea dwells. single small parietal tooth and (sometimes) Scorching sun rays act as a hard instrument of a rudimentary columellar one. selection for the speed of withdrawing. With I guess that the above explanation of these the small size of this animal (the height of its facts is related, ultimately, to the speed of re- shell is no more than 4.5 mm) the problem, traction of the snail into the shell. The species as noted above, is so acute that even a slight of the genus Euxinolauria inhabit forests, un- slowdown in withdrawing threatens snail to der condition of permanent shade; they have die because of dehydration. not a risk of rapid drying out under straight Note in passing that mollusks living in sun rays. Sometimes periods happen in the mesophitic conditions are subjected to ther- western Caucasus when the forests dry out mal shock. For example, some slugs placed very much, but the drying is so slow process under direct sun rays, may die within a few that it does not matter how long time the snail minutes, although the body of the slug can- spends for withdrawing, a few seconds or a not lose so much water during such a short few minutes. time to suggest the death due to dehydration. 4. Species of Euxinolauria living in the Perhaps, it is because of almost complete most wet habitats, viz. in the moist shady absence of the teeth in the aperture that a 292 A.A. Schileyko wide lapel of the aperture margin is devel- 6. Species living openly in the arid steppe, oped in Lauria cylindracea, due to which semi-desert and desert landscapes mostly the aperture is able to adjoin tightly to the lack teeth and a well-developed lip. In such substrate with a minimal clearance. Recall species a wide lapel of the aperture margin that provisional basal folds, periodically ap- is often developed, which allows the snails pearing in this species during shell growth to adjust tightly to the substrate. (see above) are superfi cial at the time of All the above leads to an important gen- their formation. Since a widely expanded eral conclusion. If an organism, in the course margin of the aperture can appear only at of its evolution, can substitute rigid and per- the fi nal stage of the shell growth, but the manent morphological structures for more requirements to protect the aperture do exist plastic, more fl exible and therefore more already in young snails (although not in such sophisticated physiological (or behavioral) sharp form), there is a need for some struc- acquisitions, such a replacement increases tures that are capable to temporarily replace the chances of the respective species to gain functionally the defi nitive expansion of the an advantage in a competitive interactions aperture margin. with related taxa. Morphological simplifi - The main results of the analysis of aper- cation is one of the natural results of such ture armament can be formulated as the fol- replacement. lowing six basic statements. 1. The teeth in the aperture are represent- 2.4. Axial musculature and ed by two categories that are designated here aperture armament as superfi cial and immersed. Superfi cial teeth are local extension of the lip; during shell Axial musculature consists of columellar growth they may be formed more than once. muscle and its branches. In different groups Immersed teeth are the result of a more or of Stylommatophora one can observe a dif- less noticeable narrowing of the last whorl ferentiation of columellar muscular trunk only at the fi nal stage of the shell growth. into several groups of branches to improve 2. Superfi cial teeth and lip generally serve the shell management (Suvorov, 1993). As for squeezing mucus from the mantle collar a high-spired shell requires a more sophisti- and the formation of the epiphragm. cated management than a depressed one, the 3. Superfi cial teeth are present mostly in variety of differentiation of the columellar those mollusks which bury themselves into trunk is especially noticed in members of the soil during the dry season. Enidae, Clausiliidae and other groups with 4. The main function of the immersed similar shell. Function of the shell manage- teeth is providing orientation of the shell ment is provided by the columellar lamella (with visceral sac) in respect to the cepha- in some Enidae, Chondrinidae, and some lopodium and facilitation of the management Streptaxidae,by columellar and supracolu- of a high shell. mellar lamellae and asymmetrical position 5. With increasing of aridity, a strength- of pedal branches in Orculidae, as well as by ening of the superfi cial teeth and weakening connective tissue partition which separates of the immersed ones is generally observed. the left pedal retractor of the buccal mass This trend involves the openly-living spe- from the right pedal retractor and from re- cies in which the snails survive the drought tractor of the mantle collar in Cochlicopidae by gluing with their aperture to a substrate. and Enidae (Suvorov, 1993). Characters in Stylommatophora (Mollusca) 293 3. The shell in Stylommatophora: The shell, giving a rather effective protec- an evolutionary perspective tion, limits the diversity of snail’s reactions In connection with the above attempts of to the whole broad spectrum of external irri- functional interpretation of some conchologi- tations, in essence, their reaction is reduced cal characters, it is pertinent to discuss briefl y only to the animal retraction into the shell. А slug lacking shell is forced to react more the signifi cance of the gastropod shell in a subtly, more diversely and more adequately historical aspect. to the external factors of various kinds. One can say, with some reservations, In addition, the absence of an external that it is the shell that has created a mollusk, shell facilitates penetration of mollusks into since a great number of unique morpho- narrow chinks and other hiding places. logical features characterizing the phylum Another strong argument in favor of Mollusca is associated with the shell origin. this hypothesis is the fact that the density Shell provides relatively effi cient protection of synaptic contacts in the cerebral ganglia to mollusk: 1) for most of the snails, this is of the slug Limax cinereoniger Wolf, 1803 protection predominately from predators, 2) is signifi cantly higher than that of the snail for the terrestrial ones, it additionally pro- Helix pomatia Linnaeus, 1758 (Zs.-Nagy, vides protection against water loss (what Sakharov, 1970). It follows from this com- sometimes is more important). However, the parison that the slugs, fi guratively speaking, shell, as a means of passive protection, puts are “smarter” than snails, i. e. have a more certain restrictions on the further progressive perfect nervous organization and obviously evolution of these animals. Therefore, the more complex behavior. presence of shell can be considered, from a Thus, I assume that the loss of benefi ts historical point of view, as passing phase in given by shell is compensated by a more the evolution of the terrestrial pulmonates. In complex behavior of mollusks. However, support of this suggestion, one can point to with the shell reduction the problem of gas the fact that the tendency to shell reduction exchange is exacerbated, as in this case the is observed almost wherever environmental volume of lung is decreased and the respi- conditions permit. ratory surface — the ceiling of lung cavity Among Recent taxa of Stylommato- — is reduced. This contradiction is largely phora, all stages of the shell reduction can eliminated by the fact that the skin breath- be found, from snails with well-developed ing ability is signifi cantly higher in slugs as shell to slugs, in which just conchiolin plate compared to snails (Runham, Hunter, 1970; remains (sometimes inlaid with granules of Likharev, Wiktor, 1980). Besides, additional lime) completely hidden under the mantle. respiratory surfaces are developed in some The trend towards disappearance of the shell species, such as, for example, in Vitrinidae. is absent or very weak in the ancient, most- In the terrestrial pulmonates, their man- ly morphologically archaic groups only (all tle collar is equipped initially with three pupilloid taxa, Claisiliidae, Streptaxidae + lobes (some of them may be reduced): left, Diapheridae, Megaspiridae, Urocoptidae, right and pneumostomal (= anal). The role and some others). One can assume that the of these lobes is double. First, in the time of shell gradually becomes a brake of biological withdrawal of the snail into the shell they progress of the mollusks in course of their (lobes) remove mucus from the surface of current evolution. cephalopodium; at the expense of this mu- 294 A.A. Schileyko

Fig. 14. Mantle lobes of Vitrinidae (from left to right): Phenacolimax annularis, Vi- trina pellucida, Semilimax kotulae. Рис. 14. Мантийные лопасти Vitrinidae (слева направо): Phenacolimax annulari, Vitrina pellucida, Semilimax kotulae. cus the epiphragm is formed (see above). terrestrial slugs; no doubt that they are more Second, at a gradual reduction of shell and progressive than the extinct shelled groups. formation of slug-like appearance (“limaci- Moreover, it can be argued that biological zation”) the respiratory surface is reduced, progress is determined ultimately by more and the pneumostomal lobe is greatly ex- perfect organization of the nervous system, panding (often covers a signifi cant part of the and this, in its turn, is connected with the shell), creating additional respiratory surface disappearance of external shell. The analogy and thus compensating the diminution of the between cephalopod and gastropod mollusks, lung volume. This dependence is illustrated in the given context is, to my mind, justifi ed by the morphological series in the family and demonstrative. Vitrinidae: Phenacolimax annularis (Studer, Another striking example is Opistho- 1820) (real snail with shell is well developed) branchia. Their most biologically progres- — Vitrina pellucida (Müller, 1774) (shell is sive representatives are so-called sea slugs somewhat reduced) — Semilimax kotulae (Nudibranchia) which are characterized by (Westerlund, 1883) (semislug with shell is a complete loss of shell. much reduced) (Fig. 14). With regard to the superorder Stylom- In connection with the problem under matophora, the process of reduction of the discussion, a comparison of the fossil and shell in them seems to fi t the same general contemporary cephalopods is indicative. trend. As a matter of fact, in 21 of 52 taxa The Paleozoic and Mesozoic cephalopods ranked as orders or families, there are genera had an external shell, while it became lost containing slugs or semislugs, and evolution- in their Recent relatives (completely in octo- ary reduction of the shell within the entire puses, and partly in squids and cuttlefi shes). superorder occurred (and is occurring cur- The only exception is Nautilus retaining the rently) at least 18 times (Schileyko, 2003). outer shell. In this sense, the extant cepha- In addition, two more circumstances should lopods could be regarded as analogs of the also be taken into consideration. Characters in Stylommatophora (Mollusca) 295

1. Presence of slugs and/or semislugs in at the geological time scale, the process of some taxon does not always mean that the disappearing of the shell has just begun, al- loss of shell in this group had taken place though it is gradually taking the character of only once. For example, among Succineidae a general trend. this process occurred independently at least This trend is not evident in several either twice, in South Asia and in South America highly specialized or ancient and archaic taxa (Tillier, 1981); in different branches of He- of the family rank, namely: all pupilloid taxa, licarionoidea the loss of shell also occurred Partulidae, Megaspiridae (1 species, extinct repeatedly. So the shell reduction took actu- in historical time), Urocoptidae, Clausili- ally place not in 18, but in many more num- i dae, Plectopiloidea, Strophocheiloidea, ber of cases. Cerionidae, Achatinoidea, Subulinoidea, 2. The table presented earlier by the au- Clausiliidae, Streptaxoidea, Endodontidae, thor (Schileyko, 2003, p. 149) does not in- Thyrophorellidae (1 specis), Punctidae, He- clude the taxa experiencing initial stages of licodiscidae, Discidae. the shell reduction when it is still fully devel- oped, but the fi rst signs of the limacization Acknowledgments can be seen.Thus, among Pachnodidae (in- I express my deep gratitude to Dr. A.V. fraorder Pupilloinei) where slugs and semis- Sysoev for valuable advices and improve- lugs are absent, there are genera (Amimopi- ment of my English. na, Rhachidina, Edouardia, Pachnodus), in which all or many species are characterized References by very thin walls of shell, with the number Baker H.B. 1955. Heterurethrous and aulaco- of whorls being reduced, and the last whorl pod. — Nautilus, 68 (4): 109–112. being swollen and clearly prevails over the Bizikov V.A. 2008. [Evolution of the shell in others. All this are the signs that can be in- Cephalopoda]. Moscow: VNIRO. 447 p. (in terpreted as the very initial stages of the shell Russian). Boss K.J. 1974. Oblomovism in the Mollusca. reduction. In any case, the reduction, as far as — Transactions of American Microscopic it can be seen in the factual material, begins Society, 93 (4): 460–481. with the appearance of just such symptoms. Campion M. 1961. The structure and function A similar situation is observed in Orthalici- of the cutaneous glands in Helix aspersa. — dae (genus Sultana), Helicidae (genus Can- Journal of microscopical sciences, 102 (58): tareus) and in some other families. 195–216. Within the suborder Succineiformes, rep- Edlauer E. 1941. Die ontogenetische Entwick- lung der Verschlussapparates der Clausili- resentatives of only fi ve genera and subge- iden, untersucht an Herilla bosniensis. — nera (of 28) can be allocated to the category Zeitschrift wissenschaftliche Zoologie, 155 of slugs or semislugs. But some signs of in- (2): 129–158. cipient reduction of the shell are marked in Fischer P.-H. 1971. Observations dans le milieu all genera constituting the suborder. naturel. L’etat hygrometrique de l’air influe- Same trend of gradual reduction of shell t-il sur l’activite des mollusques terrestres? in Oleacinidae can be seen in Fig. 2. — Journal de Conchyliology, 109 (1): 33–37. Likharev I.M., Wiktor A. 1980. [Slugs of the The current ratio of the shelled (snails) fauna of the USSR and adjacent countries] and shell-less (slugs + semislugs) taxa of (Gastropoda terrestres nuda). — Fauna of generic rank — 2316 against 209 (after the USSR. Molluscs. Vol. III (5). 437 p. (in Schileyko, 1998–2007) — indicates that, Russian). 296 A.A. Schileyko

Lind H. 1968. Hibernating behavior of Helix po- tropoda, Pulmonata, Geophila)]. — Fauna matia L. (Gastropoda, Pulmonata). — Vi- SSSR. Mollyuski. Т. III (3). 399 p. (in Russian). denskabelige Meddelelser Dansk Naturhis- Schileyko A.A. 1998–2007. Treatise on Recent torisk Forening. København, 131: 129–151. terrestrial pulmonate mollusks. Pts 1–15. — Ru- Moquin-Tandon A. 1855. Histoire naturelle des thenica, Suppl. 2. 2210 pp. [pt. 1 — April Mollusques terrestres et fluviatiles de France. 1998, pt. 2 — November 1998, pt. 3 — Ap- 2de partie. Paris. 646 p. ril 1999, pt. 4 — December 1999, pt. 5 — Parkhaev P.Yu. 2008.The Early Cambrian Ra- May 2000, pt. 6 — December 2000, pt. 7 diation of Mollusca. — Ponder W.F., Lind- — June 2001, pt. 8 — January 2002, pt. 9 — berg D.R. (eds). Phylogeny and Evolution of September 2002, pt. 10 — April 2003, pt. Mollusca. Berkeley: University of Califor- 11 — November 2003, pt. 12 — November 2004, pt. 13 — May 2006, pt. 14 — Novem- nia Press. P. 33–69. ber 2006, pt. 15 — June 2007]. Paul C.R.C. 1974. Azeca in Britain. — Journal Schileyko A.A. 1999. Treatise on Recent terre- of Conchology, 28: 155–172. strial pulmonate molluscs. Pt. 3. Partulidae, Pilsbry H.A. 1900. On the zoological position Aillyidae, Bulimulidae, Orthalicidae, Me ga- of Partula and Achatinella. — Proceedings spiridae, Urocoptidae. — Ruthenica, Suppl. of Academy of natural sciences of Philadel- 2: 263–436. phia, 52: 561–567. Schileyko А.А. 2003. [Directions and mecha- Raup D.M. 1966. Geometric analysis of shell coi- nisms of evolution of terrestrial pulmonate ling: general problems. — Journal of Pale- molluscs (Pulmonata, Stylommatophora)]. — ontology, 40 (5): 1178–1190. Zoologichesky zhurnal, 82 (2): 144–162. (in Raup D.M., Stanley S.M. 1971. Principles of Russian) pa le ontology. San Francisco: Freeman & Co. Schileyko A.A. 2014. On the early evolution of 390 p. pulmonate molluscs. — Arianta IV. Report Runham N.W., Hunter P.J. 1970. Terrestrial slugs. on the workshop Alpine land snails 2014. London: Hutchinson. 184 pp. Naturhistorisches Museum Wien: 13–16. Schileyko А.А. 1967а. [On the factors of vari- Schmidt A. 1855. Der Geschlechtsapparat der Sty- ability of some terrestrial mollusks]. — Vest- lommatophoren in taxonomischer Hinsicht. — nik Moskovskogo gosudarstvennogo univer- Abhandlungen des naturwissenscha ftlichen siteta. Sеr. VI (2): 14–21. (in Russian) Ve reins für Sachsen und Thüri ngen in Halle, Schileyko А.А. 1967b. [On biology of reprodu- 1 (1): 1–52. ction and juvenile adaptations of the slug Schmidt-Nielsen K., Taylor C.R., Shkolnik A. Parmacella ibera Eichw.]. — Zoologiches- 1972. Desert snails: problems of survival. ky zhurnal, 46(6): 946–948. (in Russian) — Symposium Zoological Society London, SchileykoА.А. 1975. [Peculiarities of structure 31: 1–13. Solem A., Jochelson E.L. 1979. North Ameri- of excretory apparatus of Pulmonata in the can Paleozoic land snails, with a summary connection with elaboration of the subclass of other Paleozoic nonmarine snails. — Geo- system]. — Mollyuski, ikh systema, evoly- logical Survey Professional Paper (Washin- utia i rol’ v prirode. Leningrad: Nauka. Р. gton), 1072: 1–42. 12–15. (in Russian) Steklov A.A. 1967. [Terrestrial mollusks of Neo- Schileyko А.А. 1978a. Peculiarities of the stru- gene sediments of Tuva]. — Trudy Zoolog- cture of the excretory system of the Pulmo- icheskogo Instituta AN SSSR, 42: 269–279. nata in relation to subclass classification. — (in Russian) Malacological Review, 11: 68–70. Steklov A.A., Tzуtovich M.V. 1967. [On the fi- Schileyko А.А. 1978b. [Terrestrial mollusks of nding of Neogene terrestrial mollusks in the the Helicoidea superfamily. — Fauna SSSR. Kostankol valley in Central Kazakhstan]. — Mollyuski. Т. III (6)]. 384 p. (in Russian) Byulleten Moskovskogo obstschestva ispy- Schileyko А.А. 1984. [Terrestrial mollusks of the tatelei prirody. Otd. Geologicheskoe, 42 (3): Pupillina suborder of the USSR fauna (Gas- 108–119. (in Russian) Characters in Stylommatophora (Mollusca) 297

Suvorov A.N. 1993. [Functional morphology Journal of Molluscan Studies, 47 (2): 125– of the aperture armature in the Laurii- 146. nae subfamily (Gastropoda, Orculidae) and Yom-Tov Y. 1971. Body temperature and light some problems of the systematics of the reflectance in two desert snails. — Proce- group]. — Ruthenica, 1 (1–2): 67–80. (in edings of Malacological Society of London, Russian) 39 (4): 319–326. Suvorov A.N., Schileyko A.A. 1991. [Some as- Zilch A. 1959–1960. Gastropoda Teil 2. Euty- pects of functional morphology of the aper- neura. — Handbuch der Paläozoologie, Bd. ture in the Pupillina suborder (Gastropoda 6. 834 S. Pulmonata)]. — Ruthenica, 3 (2): 141–152. Zs.-Nagy I., Sakharov D.A. 1970. The fine (in Russian) structure of the procerebrum of pulmona- Tillier S. 1981. South American and Juan Fer- te molluscs, Helix and Limax. — Tissue & nandez succineid slugs (Pulmonata). — Cell, 2 (3): 399–411.