ECOLOGY AND FUNCTIONAL SIGNIFICANCE OF UNCOILING IN VERMICULAR/A SP/RATA: AN ESSAY ON GASTROPOD FORM!

STEPHEN JAY GOULD Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138

ABSTRACT In Bermudian waters, the uncoiling marine snail Vermicularia spirata cements itself to a variety of hard substrates, predominantly to the tree coral Oculina. Uncoiling has two major functions: rapid upgrowth to attain earlier access to the rain of detrital food particles, and flexibility in growth required for stable attachment. The branching Oculina is preferred because it provides numerous attachment sites along the course of upward growth. Snails attached to Oculina begin to uncoil earlier than those ce- mented to less advantageous substrates. Comparison of the uncoiled shell with a hypothetical coiled individual of equal shell growth provides an in- dex of uncoiling. Snails attached to Oculina are more highly uncoiled than those attached to massive brain corals; snails attached to vertically oriented colonies of Oculina are more loosely coiled than those cemented to stub- bier, horizontally branched colonies of Oculina; shells fastened to the bases of colonies of Oculina are more uncoiled than those attached near the tops of the same colonies. Although many Vermicularia are very loosely coiled, none approach the theoretical optimum of uncoiling (a straight tube after attachment). Although this is due partly to the necessity for growing around obstacles, the major constraint upon complete uncoiling is the in- herited spiral mode of molluscan growth. Uncoiling is not random; Vermi- cularia always uncoils as a dextral spiral.

INTRODUCTION "God," said the Reverend Moseley (1838:354), "hath bestowed upon this humble architect the practical skill of a learned geometrician." Among molluscan architects often cited for the mathematical regularity of their coiling, Turritella occupies a prominent place. Both Moseley and D'Arcy Thompson (1942) singled out its faithful correspondence to the ideal form of a logarithmic spiral. Yet one of its descendents, the worm shell Vermic- ularia (see Morton, 1953), is so markedly erratic in the uncoiling of its later whorls that the pattern has been described as random (Abbott, 1954: 144). Two questions come to mind: What is the adaptive significance of uncoiling in Vermicularia? What is the pattern of uncoiling; is it truly random? I shall, in this work, deal with these questions as they apply to

1 Contribution No. 457 from the Bermuda Biological Station for Research. 1969J Gould: Uncoiling in Vermicularia 433 the common North American worm she]], Vermicularia spirata (Philippi, 1836), in Bermudian waters. Most snails are mobile, benthic browsers, feeding on plant material scraped up by the toothed radula. The choice of actively searching for food as a mode of life requires that the snail's long body be housed in a compact, relatively streamlined shell. In the absence of shell segmentation, regular coiling is the most efficient way to fulfill this requirement. It is not surpris- ing, therefore, that most immobile snails depart from the usual patterns of coiling. The filter-feeding capulid and calyptraeid "limpets" (including the slipper shell Crepidula) have not become irregular in their coiling, but they expand the generating curve so rapidly that very few whorls form before adult sizes are reached. Another departure, uncoiling, has been developed independently by several groups of filter-feeding gastropods, often for dif- ferent reasons. (See Yonge, 1937, on the relationship of filter feeding and sessHity.) Many of the intertidal true vermetids cement to rocky shores in the wave zone and uncoil erratically in order to provide a maximum sur- face for attachment. Several siliquariid species live in sponges; their loose coiling produces the rapid axial translation that may be necessary to keep up with the growth of their host (W. J. Clench, personal communication). I propose that loose coiling in the turritellid Vermicularia spirata also has rapid growth along the axis as its primary adaptive significance. But the impetus for rapid growth is different here; V. spirata cements its shell to a variety of hard substrates and grows upwards. Rapid upgrowth provides a dual advantage: earlier access to the rain of detrital particles that consti- tute the food supply and quick elevation above bottom sediment that might clog the ciliary feeding apparatus. Yet loose coiling is not the only require- ment for success in cementing forms; flexibility is necessary so that ob- stacles in the course of upgrowth can be circumvented and additional points of attachment secured when necessary. A loosely, but extremely regularly, coiled genus such as Laxispira (often incorrectly regarded as a subgenus of Vermicularia, e.g., by Cossmann, 1912) is far too controlled in its direc- tions of growth for success in such an environment. Vermicularia spirata thus combines the rapid directional growth of siliquariids with the flexibility for continual attachment of vermetids. Such requirements may seem con- tradictory, for horizontal growth to secure attachment should operate in opposition to vertical upgrowth. The potential conflict can, however, be resolved by a suitable choice of substrate, one which provides numerous sites of attachment along the course of upward growth (see Fig. 1).

NATURAL-HISTORY OF V. spirata During the summers of 1964-1967, I studied the morphology and mode of life of Vermicularia spirata in Bermudian waters. This common species 434 Bulletin of Marine Science [19(2)

FIGURE 1. A specimen of Vermicularia spirata attached to a vertically oriented colony of Oculina (initial turritellid stage at A, adult aperture at B). Note the availability of numerous attachment sites along the course of upward growth, allowing stability to be maintained with rapid upgrowth. 1969] Gould: Uncoiling in Vermicu/aria 435 lives both in protected bays and lagoons (Walsingham Pond, Harrington Sound) and in subtidal open-sea habitats. Its range of depth is large. In Harrington Sound, for example, V. spirata inhabits all depths from inches below extreme low tide to the maximum of 82 feet (Neumann, 1965). Its temperature tolerance must also be great, for it is common throughout the West Indies and has been reported as far north as Buzzards Bay, Massa- chusetts (Stimpson, 1851). The protoconch of Vermicularia spirata is reduced in comparison with that of Turritella species having a free-swimming veliger; Morton (1953: 84) thus supposed the veliger stage to be suppressed in Vermicularia. The foot, functionless and reduced in the attached adult, is small but functional in the regularly coiled turritellid juvenile stage. The maximum length of the extended juvenile foot is never more than Y:{ the length of the shell. Movement is most inefficient compared with that of most browsing snails. In poorly oxygenated tanks, juveniles of V. spirata will climb up the side walls of the aquarium. The juvenile moves in a series of small steps rather than in a continuous progression. The foot is extended and the animal moves forward out of its shell; the shell is then pulled up to the new level, but it falls slightly backwards before the sequence is completed. I timed sixty such sequences for an individual whose shell was 15.4 mm long and whose extended foot had a maximum length of 4.5 mm. The average for- ward motion was 1.65 mm per sequence (36 per cent of the foot's length) and a sequence averaged 21.4 seconds. When placed in a sandy-bottomed aquarium tank far from a potential uncoiling site (the coral Ocu/ina), individual juveniles either form a Tur- ritella-like feeding burrow in the sand or crawl to the coral and lodge them- selves in a stable spot thereon. I have seen them resting immobile on the coral, though presumably capable of movement, for up to two weeks be- fore physically cementing to begin uncoiling. Either in the sand burrow or on the coral, they filter feed, as does the adult, by passing water currents in and out between the operculum and the shel1 margin. The operculum is held at, or just slightly in front of, the shell margin at all times unless the animal is disturbed, at which time the operculum is sharply retracted far into the shell. The diameter of the operculum is slightly smaller than that of the aperture, but the difference is not enough to account for the deep retraction of the animal. This is permitted by the extremely flexible oper- cular edge which lacks the supporting bristles of Turritella.

THE COILED JUVENILE SHELL The most outstanding aspect of Vermicularia's distribution in Bermudian waters is its preference for the tree-coral Oculina as a substrate for cemen- tation and subsequent uncoiling. I have never seen a large accumulation 436 Bulletin of Marine Science [19(2) of colonies of Oculina without attached Vermicularia; often every coral colony is host to many uncoiled snails. Where Oculina is not found, Ver- micularia is rare or absent (with the single exception of Walsingham Pond [see Gould, 1968], where large numbers have no sites for attachment). If the hypothesis of the introductory section is accepted, then this striking cor- relation can be explained. If rapid upgrowth with secure attachment is advantageous, then Oculina is unique among potential substrates in pos- sessing a vertically oriented, branching structure which provides numerous attachment sites along a potential course of upward growth. Indeed, the only other possibility would be the loosely spiralled shells of other individ- uals of Vermicularia. I have seen museum specimens, purportedly from Bermuda, of colonies of Vermicularia in which 50 to 100 shells, their aper- tures all pointing upwards, form an intertwining network of highly uncoiled tubes. Data on variation in length of the juvenile coiled shell confirms the pref- erence for Oculina as an attachment site, for mean length of the turritellid stage is shortest in Oculina-based local populations. I found a continuum that ranged from the 5- to 6-mm coiled shells in patches of Oculina at North Reef to shells of the remarkable local population at Walsingham Pond, the majority of which did not uncoil at all. Walsingham is a landlocked marine pond with a muddy bottom devoid of coral or any hard substrate; here fewer than 1 per cent of the largest individuals of V. spirata show any sign of uncoiling. They live in burrows, their apertures projecting slightly above the mud; those that do uncoil lack an attachment scar and unwind slowly and rather regularly. Here, in the type of environment favored by TurriteIla (Yonge, 1946), they have reverted to the habits and to the form of their ancestor (Gould, 1968). Coiled portions of intermediate length are found in areas devoid of Ocu- lina, but furnishing potential substrates which lack the advantageous verti- cal orientation and branching form of tree corals. Massive "brain corals" of the genus Dip/oria and stubby colonies of Porites are occasionally used as attachment sites in various areas of the north reefs. A few Vermicularia were found on Area zebra shells in a "mussel" bed of Harrington Sound. Twenty-two specimens were collected from a single sponge dredged from Three Hills Shoals off the northern coast. A single individual uncoiled along the platy frond of Udotea in an algal bed north of Whalebone Bay. Table I summarizes the data on lengths of juvenile coiled shells in sev- eral Bermudian localities. Available substrate is the primary determinant of this character, but a relationship to bottom turbulence is also indicated. Of Oculina-based juvenile shells, those from the calm waters of Harrington Sound are longest; all samples from the northern reef tracts and Castle Harbor come from areas of reasonably strong tidal currents. In Harrington 11)69] Gould: Uncoiling in Vermicularia 437 TABLE 1 RELATION OF HABITAT TO LENGTH AT UNCOILING IN V. spirata MeanlengthStandard Number at uncoilingdeviation of Locality Substrate (mm) (rom) specimens Attached to Oculina Castle Harbour Oculina 5.00 1.49 21 Three Hills Shoals Oculina 6.40 1.62 22 North of Whalebone Bay Oeulina 7.72 1.77 35 Harrington Sound Oeulina 8.87 2.14 27 Not Attached to Oeulina Various North Reef localities Brain corals 8.72 1.68 6 North of Tobacco Bay Sponge 9.01 1.37 22 North of Whalebone Bay Alga (Udotea) 12.53 1 Harrington Sound Clam (Area) 12.11 1.82 7 No Available Attachment Site Walsingham Pond Mud 18.16 4.63 31

Sound, specimens attached to substrates other than Oculina are, likewise, longer than corresponding samples from reef habitats. (Among all Har- rington Sound specimens, those attached to Oculina are significantly smaller than those uncoiling upon other substrates: t = 3.7 at 32 d.f. The primary relationship of size and substrate is again affirmed.) I see no reason to ascribe these differences to genetic modification in iso- lated local populations, for most of the samples are from contiguous areas off Bermuda's north shore. Moreover, shells from the morphologically most unusual and physically most isolated population at Walsingham Pond can be induced to uncoil at normal lengths for Oculina-based popu- lations by providing such an environment in a laboratory tank. The genetic determinants of uncoiling permit wide flexibility in the onset of this charac- teristic; the stimulus of a favorable substrate can induce early uncoiling. In extreme cases, most members of a population do not uncoil at all. Yet Ihis flexibility might have an upper limit in size, for the Walsingham ani- mals which never uncoil die at a much smaller size than that of most un- coiled individuals in Oculina environments. Moreover, V. spirata shells from a mud flat in Tampa Bay, Florida, which do reach normal adult sizes, have a coiled portion equal in length to the shells at Walsingham Pond, but do uncoil slowly and regularly at larger sizes. (These were improperly constituted as a separate species, Vermicularia fargoi, by Olsson, 1951; see Gould, 1968.) PATTERNS OF UNCOILING We would like to test the hypothesis that rapid upgrowth is the adaptive reason for uncoiling in V. spirata and, further, that attachment to Oculina favors such growth. A good measure of gain in upgrowth would be fur- 438 Bulletin of Marine Science [19(2) nished by comparison of an uncoiled shell with a hypothetical regular coil produced by continuation of the initial turritelloid juvenile growth through- out ontogeny. Fortunately, the close correspondence of juvenile growth to the ideal form of a logarithmic spiral permits the construction of this hypo- thetical regular coil. The logarithmic spiral is the only curve which in- creases by terminal accretion without alteration of shape (Thompson, 1942). Since dimensionless properties (angles and ratios for example) of geometrically similar structures are invariant, we may extrapolate ratios measured on juvenile coiled shells to construct the hypothetical coiled adult. As a first step, 1 tested the correspondence between actual juvenile shells and the logarithmic spiral. 1 wrapped a fine thread around the shell be- tween the two prominent spiral ornaments and marked with an ink dot the position of the last four whorls before uncoiling. Unrolling the thread, 1 measured the lengths of the four whorls and computed the ratios (three for each shell) of the length of a whorl to the length of the preceding whorl. If growth conforms to a logarithmic spiral, these values should be invariant for any given shell. For nine shells measured, mean values of the three ratios (length of last whorl before uncoiling/length of next to last, next to last/second from last, and second from last/third from last) were 1.292, 1.284, and 1.288, respectively. As a further consequence of logarithmic spiral growth, the ratio of any measure with its corresponding measure on the previous whorl should have the same value as that of the ratio of the lengths of the two whorls. 1 mea- sured the heights of the same four whorls in each of the nine specimens that were used in the analysis of whorl-length, and calculated ratios in the same manner. The mean of all 27 ratios of heights is 1.296 with a stan- dard deviation of 0.034, while that for the 27 ratios of lengths is 1.288 with a standard deviation of 0.045. (I imagine that the higher standard devia- tion in whorl-lengths is an artifact of greater error of measurement in the string-winding technique vs. direct measurement technique). The difference between means is insignificant at any commonly used significance level (t = 0.74 at 52 d.f.). Assuming that the coiled juvenile portion can be treated as a logarithmic spiral, it follows that the ratio of height and length for each whorl will be constant throughout the turritelloid growth of an individual, i.e., (1) whorl-length = c (whorl-height) where c is a constant. This equation can then be used to construct a hypo- thetical coiled shell corresponding to any individual uncoiled V. spirata. The value of c in the above equation is determined by using the measure- ments for heights and lengths of coiled whorls. Continuing the winding of string along the uncoiled portion and extrapolating the observed ratio of whorl-lengths in the coiled portion, we can calculate the number of whorls 1969] Gould: Uncailing in Vermicularia 439 of a fully coiled shell that would be equal in total length to the uncoiled shell. With equation 1, this value can be used to determine the total height of this hypothetical coiled shell.2 The hypothetical coiled shell is a refer- ence for determining the role of uncoiling in producing rapid upgrowth, for the ratio of total height of the uncoiled specimen to total height if the speci- men had not uncoiled is an index of the gain in height realized by uncoiling. I used this method of analysis to make three comparisons: ( 1) between specimens attached to Oculina and those cemented to the brain coral Diploria; (2) between specimens attached to the base of vertically oriented colonies of Oculina and those cementing to the base of more horizontally branched colonies of Oculina; and (3) between specimens attached to the base of a vertically oriented colony of Oculina and those attached else- where on the same colony. Comparison i.-For six specimens attached in various positions on colonies of Oculina of varying form, the ratio of total height to total height if not uncoiled ranges from 0.93 (for an individual attached near the top of a colony) to 3.20 (Fig. 2, A). For two specimens cemented to colonies of Dip/aria, the ratios are 0.15 and 0.65 (Fig. 2, C). The smooth hemispher- ical form of colonies of Diploria provides no support for individuals grow- ing upwards. Compelled to spread laterally along the coral surface, speci- mens of V. spirata attached to Dip/oria are not as high as a hypothetical coiled shell of equal length. Comparison 2.-Specimens of V. spirata are concentrated on the large, vertically oriented colonies of Oculina of the north reefs and deeper parts of Harrington Sound. The largest amount of uncoiling (ratios of 2.00 and 3.20) occurred in specimens attached to the bases of such colonies (Fig. 2, A, B). In the short, delicate, more horizontally oriented colonies of Oculina common in the shallow water of Harrington Sound near Church Bay, basally attached specimens of V. spirata are not as strongly uncoiled (ratios of 1.15 and 1.17 in two specimens measured). Differences in de- gree of uncoiling seem to be mechanically related to the form of the colo- nies of Ocu/ina that act as attachment sites. The close correspondence of a highly uncoiled specimen with the vertical branching of its host is shown in Figure 1. Comparison 3.-0ccasionally, specimens of V. spirata are found attached at several levels of a large Oculina colony. The specimen in Figure 2, B (ratio 2.00) was attached to the base of such a colony. Another, initially cemented at about % the distance from base to top yielded a ratio of 1.88, while a specimen beginning its uncoiling closer to the top than to the base

• The potentially simpler method of directly measuring and summing the whorl heights at each point of the uncoiled shell marking a whorl of the hypothetical coiled shell is not accurate because the shape of the whorl changes upon uncoiling. The quadrate aperture of the coiled portion becomes almost perfectly circular. 440 Bulletin of Marine Science [19(2) of the colony took a pronounced horizontal meander and ended up as the only Oculina-based specimen lower in final height than the corresponding regular coil (ratio of 0.93). I imagine that these differences indicate a faster growth rate of the snail than the coral. The vertical orientation of coral branches persists in the upper part of the colony; pronounced hori- zontal growth of a V. spirata beginning to uncoil in this upper part may result when the snail, growing much faster than the coral, reaches the top of the colony and spreads laterally in the absence of further support for upgrowth.

CONSTRAINTS UPON UNCOILING In addition to comparing an actual shell with a corresponding regular coil, we might usefully contrast it with the hypothetical structure created by maximal upgrowth, i.e., with a snail which, after uncoiling, grew up- wards as an absolutely straight tube as long as that of the actual specimen. If the regular coil is ranked as 0 in height and the straight tube as 100, the ranking of the actual specimen may give some indication of its approach to optimality. In using the word "optimality" I am proposing that maximal upward growth is ecologically the most adapted condition, but that the snail is prevented from achieving this state by several constraints. We have been discussing constraints of an ecological nature, in particular the necessity for numerous attachment sites along the course of upward growth. The horizontal spreading that assures such attachment prevents any really close approach to 100 per cent uncoiling. Another constraint detracts from maximal uncoiling in several specimens. Most individuals of V. spirata attach to Oculina with their apices down, at an angle of approximately 10°. (It cannot be entirely coincidental that this is the angle assumed by Turritella shells in their burrows-see Yonge, 1946). Occasionally, a shell will attach apex upward. Before upward growth can begin, this individual must "right itself" by producing a mark- edly irregular whorl which limits (however slightly) the opportunity for maximal uncoiling. That such shells do not grow downward is a further argument for the idea that rapid upward growth is the primary advantage of uncoiling. The higWy uncoiled shells on vertical colonies of Oculina are subject in no great degree to either of these constraints (Fig. 2, A, B). Attachment sites for a nearly straight tube would be sufficient and the only constraint might be the necessity for growing around an occasional horizontal branch. We might therefore expect a rather close approach to optimality in these specimens. Yet this does not occur. The straightest specimen I have ever seen (Fig. 2, A) ranks only 48; it has not even reached the halfway point between regular coil and straight tube. Clearly a constraint even more serious than those previously considered is acting here, and this can only 1969] Gould: Uncoiling m Vermicularia 441

A c FIGURE 2. Specimens of Vermicularia spirata from Bermudian corals: A, for- merly attached to the base of a vertically oriented tree coral, Oculina. Ratio of actual height to theoretical height if regularly coiled is 3.20, yet the specimen ranks only 48 on a scale ranging between the height of the theoretical regular coil (ranked 0) and the height of a maximally uncoiled shell (a straight tube after uncoiling, ranked 100). B, formerly attached to the base of a vertically oriented tree coral, Oculina. Ratio of actual height to theoretical height if regu- larly coiled is 2.00. C, formerly attached to the brain coral Diploria. No at- tachment points were available for upgrowth, and the individual spread hori- zontally. Ratio of actual height to theoretical height if regularly coiled is 0.65. 442 Bulletin of Marine Science [19(2) be the spiral mode of gastropod growth. Spiral growth is the most perva- sive feature of molluscan shells. Even the "straight" tubes of scaphopods and the gastropod genus Caecum are actually gentle curves which, as D'Arcy Thompson showed (1942), are logarithmic spirals with unusually low spiral angles. No matter how loose the coil, no matter how rapid the upgrowth, V. spirata always uncoils as a dextral spiral. The ecological ad- vantages of upward growth are counteracted by the genetic constraint of spirality. Within the genetic system of V. spirata, the form of a maximally uncoiled shell is a very loose dextral spiral. The constraint of spirality is seen more forcefully in the colonies of V. spirata that I have seen in several museums. These are solid masses of shells in which the only attachment site is another V. spirata. No ecologi- cal factor prevents nearly straight upgrowth here, not even a necessity for minimal separational distances between individuals (since no spaces remain other than those geometrically unavoidable when circular apertures of rela- tively constant size are pressed together). Yet the tubes are not nearly straight, but are, instead, complexly twined about one another in dextral spirals. Figure 3 illustrates another aspect of regularity in growth of the uncoiled portion. This figure shows a plot of the height of each successive whorl vs. the number of that whorl. (The number of the whorl in an uncoiled shell is an extrapolation from the coiled portion and represents the place at which that given whorl would have been completed if the shell had continued its regular juvenile coiling). The abrupt rise in position of the line at whorl five marks the transition to uncoiling when the quadrate aperture of the coiled juvenile expands rapidly to the circular form of uncoiled shells. The slope of both sections of the curve is the same and the regularity of increase in whorl-height of the uncoiled portion is particularly striking. Despite the erratic path of upgrowth and numerous attachment scars, whorl-height of the uncoiled shell increases with the same regularity as that of the coiled portion. Moreover, it increases at the same rate, indicating that uncoiling does not disturb the precise regularity of increase so characteristic of nor- mal snails which take the logarithmic spiral as their model. In conclusion, the uncoiling of Vermicularia spirata is anything but ran- dom. Irregular coiling provides the flexibility which allows a sessile animal to grow around obstacles, but this irregularity is counteracted by directing tendencies which can be divided into two categories: those due to' specific environmental adaptations and those attributed to more general character- istics of the biological group to which Vermicularia belongs. This division corresponds to the distinction that W. K. Gregory often made between the factors of habitus and heritage (e.g., Gregory, 1913, 1922). The advan- tages of rapid upgrowth belong to the first category while spiral growth, 1969] Gould: Uncoiling in Vermicularia 443

2.0

1.8

1.6 •... :r: Q w 1.4 :r: ...• '"0 :r: 1.2 ~

1.0 2 3 4 5 6 7 8 9 10

WHORL NUMBER FIGURE3. Plot of height of whorl vs. the number of the whorl for the specimen in Figure 2, B, starting with the fourth whorl before the beginning of uncoiling and using the hypothetical coiled shell as a criterion for determining the posi- tions of the whorls on the uncoiled portion. Note the striking regularity of the uncoiled portion with the slope of the line equal to that of the line for the coiled juvenile. The change from quadrate to circular aperture at the begin- ning of uncoiling causes the change in position of the line. perhaps the most fundamental feature of the molluscan shell, prevents any close approach to randomness by imposing upon V. spirata the primal con- straint of its class.

ACKNOWLEDGMENTS I thank K. E. Chave and R. F. Schmalz for making observations and collecting specimens from the deep waters of Harrington Sound. G. O. G. Greiner, G. J. Brunskill, and A. C. Neumann brought new collecting locali- ties to my attention and supplied much sound advice.

SUMARIO ECOLOGIA Y SIGNIFICACION FUNCIONAL DEL DESENROLLADO DE LA CONCHA EN Vermicularia spirata: UN ENSAYO SOBRE UNA FORMA GASTEROPODA Los gaster6podos sesiles que se alimentan por filtraci6n casi siempre muestran patrones an6malos de enrollado de la concha. En aguas ber- mudenses, el territelido Vermicularia spirata es normalmente enroll ado y m6vil cuando joven, pero mas tarde en su vida se pega a un substrato duro y se desenrolla. 444 Bulletin of Marine Science [19(2) Un nlpido crecimiento hacia arriba es la mayor ventaja al desenrollarse Vermicularia. Provee un temprano acceso a la lluvia de alimentos detriticos y nlpida traslaci6n lejos de los fondos fangosos que pueden tupir el aparato de fiItrar los alimentos. Una segunda ventaja es flexibilidad, la que Ie per- mite el crecimiento alrededor de obstaculos y asegura numerosos sitios de fijaci6n para mayor estabilidad. El coral ramificado Oculina es abrumadoramente preferido como sub- strato para fijarse. Es el unico coral de Bermudas que puede proveer varios lugares de fijaci6n en el curso de un crecimiento hacia arriba de un enro- llado sueIto. La preferencia de Oculina se muestra por 10 siguiente: (a) los caracoles adheridos a Oculina se desenrollan cuando son mas pequefios que los pegados a otros substratos; (b) un Indice del desenrollado se obtiene com- parando la concha actual con una concha enroll ada te6rica de una espira1 de longitud similar. Las conchas adheridas a Oculina estan mas enrolladas que las adheridas a masas de corales cerebros. Entre las colonias de Oculina, los caracoles pegados a estructuras verti- calmente orient adas estan enrollados mas flojos que aquellos asociados con corales mas pequefios, horizontalmente ramificados. Dentro de una colonia de Oculina, las conchas pegadas a las base de una colonia estan mas desen- rolladas que aquellas pegadas mas cerca del extremo superior. Varias restricciones impiden que las conchas de Vermicularia alcancen la forma te6rica de maximo crecimiento hacia arriba (un tubo recto). El crecimiento alrededor de los obstaculos es una restricci6n de habitus. La limitaci6n mas importante es la de la herencia-la forma de crecimiento en espiral de los gaster6podos. Vermicularia siempre se desenrolla como una espiral derecha. Ademas, el aumento en altura de la espiral es notable- mente uniforme en la concha desenrollada y ocurre en la misma proporci6n que en los juveniles enrollados. El desenrollado en Vermicularia no es fortuito sino que esta sujeto a largas series de controles geneticos y ambientales. REFERENCES ABBOTT, R. T. 1954. American seashells. D. Van Nostrand Company, Inc., N. Y., 541 pp. COSSMANN, M. 1912. Essais de palt~oconchologiecomparee. Volume 9. J. Lamarre, Paris, 215 pp. GOULD, S. J. 1968. Phenotypic reversion to ancestral form and habits in a marine snail. Nature, 220: 804. GREGORY, W. K. 1913. Locomotive adaptations in fishes illustrating "habitus" and "heritage." Ann. N. Y. Acad. SeL, 23: 267-268. 1922. On the "habitus" and "heritage" of Caenolestes. Jour. Mammalogy, 3: 106-114. 1969] Gould: Uncoiling in Vermicularia 445

MORTON, J. E. 1953. Vermicularia and the turritellids. Proc. malacol. Soc. London, 30: 80- 86. MOSELEY, H. 1838. On the geometrical forms of turbinated and discoid shells. Phil. Trans. roy. Soc. London, 128: 351-370. NEUMANN, A. C. 1965. Processes of recent carbonate sedimentation in Harrington Sound, Ber- muda. Bull. Mar. Sci., 15: 987-1035. OLSSON, A. A. 1951. New Floridan species of Ostrea and Vermicularia. Nautilus, 65: 6-8. STIMPSON, W. 1851. Shells of New England. Phillips Sampson and Company, Boston, 56 pp. THOMPSON, D. W. 1942. On growth and form. Cambridge University Press, Cambridge, En- gland, 1116 pp. YONGE, C. M. 1937. Evolution of ciliary feeding in the Prosobranchia, with an account of feeding in Capulus ungaricus. Jour. Mar. BioI. Ass. U. K., 22: 453- 468. 1946. On the habits of Turritella communis Risso. Jour. Mar. BioI. Ass. U. K., 26: 377-380.