ON THE STRUCTURE AND MECHANISM OF THE GASTRIC MILL IN .

IV. The Structure of the Gastric Mill in Reptantous Macrura.

By S. S. PATWARDHAN, M.Sc., Department of Zoology, College of Science, Nagpur.

Received January 25, 1935. (Communicated by Prof. M. A. Moghe, M.A., M.Sc., r.z.s.) 1. Introduction. THE sub-order Macrura may be divided into two groups: (1) Reptantous Macrura, comprising , and allied forms and characterised by possessing a dorsoventrally flattened body and a crawling and climbing mode of locomotion, and (2) Natantous Macrura comprising and , characterised by possessing a laterally flattened body and a swimming mode of locomotion. The Reptantous Macrura are also characterised by the universal presence of a complex gastric mill and simple mandibles. In the present communication it is proposed to give a comparative account of the gastric mill in Reptantous Macrura. 2. Material and Method. The gastric mills of , Astacus fluviatilis, Palinurus and of many other lobsters have been already described in many text-books (Huxley, 1880; Powell, 1913) . The material at my disposal consists of six obtained from various Biological Supplies Stations and represents all the tribes, excepting Eryonidea, and four families. Tribe Family Nephropsidea N ephropsidae Homarus vulgaris* M. Edw. Nephrops norvegicusf Leach. Loricata Palinuridee Palinurus vulgaris* Latr. Scyllaridee arctus Pabr. Thalassinidea Callianassidae Callianassa subterraneat Leach. Gebia littoralist Desm. Two or three specimens of each type were examined and were treated with Caustic Potash as I did in Anomura. A brief account of the cardiac

* From Plymouth. t From Naples. 414 Gas/nic Mill in Decapoda 415 and the pyloric stomach and a comparative account of the principal ossicle of the gastric mill is given below. 3. The Cardiac and the Pyloric Stomach. The cardiac stomach is usually a subspherical sac or it may be slightly elongated anteriorly. It is always flattened from above and this flattening is correlated with the dorso-ventral depression of the body of the lobsters. The paired ventral grooves, limiting the floor of the cardiac stomach from the sides, are always present. The comb-like rows of thick setae covering these grooves subserve a similar function as in Anomura

and Brachyura. The lateral accessory teeth are absent in Scyllarus' and Palinurus. In Nephrops (Pig. 1, l.a.t.) and Homarus each tooth consists

1 In Scyllarus, there is a short tooth-like process on the anterior edge of the zygocardiac ossicle which perhaps represents as in Hippa (Patwardhan, 1935), the lateral accessory tooth. 416 S. S. Patwardhan of two or three small spines. In Callianassa (Fig. 3, l.a.t.) it consists of a triangular plate bordered with short spines—a condition usually met with in Anomura. The cardiac pyloric valve usually consists of a pair of lobes crowned with set, e.g., in Nephrops, Palinurus (Figs. 1 and 2, c. .v.), Homarus and Scyllarus. In Callianassa (Fig. 3, c.p.v.) and Gebia, the lobes of the cardiac pyloric valve bear teeth-like cuticular outgrowths deeply pigmented brown or yellow---a condition characteristic of Anomura. The pyloric stomach is narrow and constricted off from the cardiac stomach. It is usually not broader than long, except in Scyllarus in which it is slightly elongated. The ossicles developed in the wall of the pyloric stomach are not thick and calcified to the same extent in all the types examined by me. In Palinurus and Scyllarus they are thin, while in

the remaining forms they are thick and densely calcified. The pyloric stomach is divided into two chambers by a pair of lateral folds: (1) a dorsal chamber, and (2) a ventral chamber. The latter chamber contains the characteristic filtering apparatus. The opening between the pyloric stomach and the midgut is guarded by four valves arranged as follows: (a) a dorsal Gasiric Mill in Decc oda 417 median, (b) a pair of latero-ventral from the wall of the dorsal chamber and (c) a median ventral, which is a backward prolongation of the inter-ampullary ridge of the filtering apparatus. In Callianassa and Gebia the dorsal median valve is reduced and there is an additional latero-dorsal pair of valves, one on each side of the dorsal chamber. These valves are usually elongated, directed backward and covered over with elongated set. In Palinurus, the valves are flattened and broad and in Callianassa and Gebia, the latero-dorsal valves are broken up into a number of long strands. 4. The Gastric Mill. In all the types examined by me, the gastric mill is well developed and consists of the same typical ten ossicles as in Brachyura and Anomura. (a) Ossicles of the anterior arch.—As in Anomura, the mesocardiac ossicle forms the largest portion of the anterior arch. It is semicircular in Nephrops, Callianassa (Figs. 1 and 3, m.c.), Homarus and Scyllarus or triangular as in Palinurus. (Fig. 2, m.c.). In Gebia, it is a transverse oblong plate. Laterally the base of the ossicle presents on each side a small chink where it is articu- lated with the pterocardiac ossicle of its side. Correlated with the large size of the mesocardiac ossicle are the small pterocardiac ossicles. Each is a curved triangular plate articulating with the mesocardiac ossicle along the broad base, while its bent apex is connected to the anterior process of the zygocardiac ossicle. Such a typical arrange- ment is found in Nephrops, Callianassa (Figs. 1 and 3, fit.c.), Homarus and Gebia, but in Palinurus (Fig. 2, 75t.c.) and Scyllarus these ossicles are slightly elongated and straight. The relative sizes of these two ossicles, viz., nuesocardiac and pterocar- diac ossicles, are the same as in Anomura (Patwardhan, 1935) and also as in Natantous Macrura to be described in a subsequent paper. (b) Ossicles of the posterior arch.—The pyloric ossicle is a flattened curved plate forming the anterior portion of the roof of the pyloric stomach. Its posterior border is a simple curve, e.g., Nephrops, Palinurus (Figs. 1 and 2, .P.) and Scyllarus. In Homarus, it is drawn backwards into a conical projection and thus gives the ossicle a triangular appearance. In Palinurus (Fig. 2, p.), the middle and the posterior region of the ossicle is thin and less calcified, while in the remaining forms the calcification is more or less uniform. The exopyloric ossicles are not distinctly marked off from the pyloric ossicle. But their size and shape can be fairly determined by following the discon- tinuous anterior margin of the pyloric ossicle and the deficiency of calcification of the membrane connecting the exopyloric and the pyloric ossicle. In Nephrops and Callianassa (Figs. 1 and 3, ex. jb.) and Homarus the 418 S. S. Patwardhan

FIG. 4. Gastric Mill in Decapoda 419 exopyloric ossicles appear to be irregularly four-sided, while in Palinurus (Fig. 2, ex.p.) and Scyllarus they are triangular. The zygocardiac ossicles are typically three-sided and each projects from the side into the cavity of the cardiac stomach. Laterally each ossicle presents a long anterior process which is connected to the outer end of the pterocardiac ossicle of its side. Posteriorly there is a small vertical process which articulates with the exopyloric ossicle. The median edge of the ossicle is modified into a lateral tooth. The latter consists of one, e.g. , Palinurus, Callianassa (Figs. 2 and 3, l.t.) and Scyllarus, or two, e.g., Ne^hrops (Fig. 1, Lt.) and Homarus, stout anterior denticles followed by a row of vertical ridges of varying thickness. (c) Ossicles connecting the two arches. --The anterior urocardiac ossicle is platelike, with its long axis lying obliquely along the median longitudinal plane. Its anterior portion is broad and gives the ossicle a bell-shaped appearance as in Nephroj5s, Callianassa (Figs. 1. and 3, u.c.) and Scyllarus or a triangular one as in Palinurus (Fig. 2, u.c.). The posterior half of its inferior surface bears the median tooth which shows some interesting modi- fications. In Nephroos (Fig. 1, m.t.) and Homarus it consists of a single median U-shaped tooth with a pair of small denticles in front of it. In Palinurus (Fig. 2, m.t.) and Scyllarus the median tooth is thin and bifid and is folded upon itself anteriorly. The sides of the urocardiac ossicle leading upto the folded median tooth are marked with indentations. In Callianassa (Fig. 3, m.t.) and Gebia the median tooth is heart-shaped and elliptical respectively and in both the sides of the tooth are sloping and carved into thin transverse ridges. The lateral as well as the median teeth are deeply pigmented brown or yellow. The prepyloric ossicle may be, more or less, a T-shaped plate, e.g., Callianassa (Fig. 3, pr.p.) or Y-shaped, e.g., Palinurus (Fig. 2, r.p.) and Scyllarus. In Nephrops (rig. 1, pr.p.), Homarus and Gebia it is more or less triangular with curved edges. The base of the triangular plate or the transverse portion of the differently shaped ossicle is connected to the pyloric ossicle while the apex of the triangular or the lower end of the longitudinal portion of the T- or Y-shaped ossicle is always bifid and is connected to the posterior end of the urocardiac ossicle. 5. Feeding Mechanism.. Feeding mechanism of the crayfishes and lobsters is well known. The elongated antennae explore the neighbourhood by whipping water in different planes to locate the food material by the sense of smell. The food is either caught by the antennae or the pincer claws and if the food is far away the leaves its abode, usually a crevice in rock or a burrow in sand, captures its prey, transfers it to the max.illip,eds and quickly returns 420 S. S. Patwardhan to its place of shelter (Williamson, 1904). The larger claws are very powerful and are used for crushing or breaking open shells, while the smaller ones act as prehensile organs and are used to tear the food material into suitably sized morsels and to transfer them into the mouth. As in Anomura and Brachyura the mandibles are remarkably simple in structure. The `° head " portion bears the incisor process which may have a bluntly toothed edge smeared with a yellow or brown pigment, e.g., Nephrops, Hoznarus, Palinurus, Scyllarus and Gebia (Pig. 4, A, B, C, D and F) or the teeth may be very prominent and deep as in Callianassa (I). The molar prominence is altogether wanting in Scyllarus. In Palinurus and Callianassa it is represented by a simple ridge, while in Homarus, Nephrops and Gebia it is round or oval in shape. A three-jointed palp, playing on the superior surface of the incisor process is always present, excepting in Scyllarus (Fig. 4, D) where it is very much reduced. The structure of the mandibles thus indicates that the mastication of food does not take place to any appreciable extent in the buccal cavity.

6. Discussion. While explaining the presence of a complex gastric mill and the consequent presence of simple mandibles in Anomura and Brachyura it was pointed out that there were reasons to believe that Anomura and Brachyura are derived from -like ancestors which possess gastric mill. It is also believed that crayfishes are derived from marine lobsters and that "lobsters are derived from -like swimming forms which have sacrificed some of their agility in developing their heavily armoured plating..... (Calman, 1911) . The beginnings of gastric mill are already found in some Natantous Macrura, e.g., Peneidea and hence it may be said that as in Anomura and Brachyura, the gastric mill is a legacy from the ancestors. The absence or presence of a gastric mill in the cardiac stomach depends upon whether the process of mastication of food is to be completed before or after the act of swallowing the morsels of food thrust into the buccal cavity. It seems that a change from a swimming mode of locomo- tion to that of a creeping and a climbing one necessitates transference of mastication after the act of swallowing from the mandibles in the buccal cavity to the gastric mill in the cardiac stomach. The change of the mode of locomotion has placed these in new pathways of food, new environment and new enemies. They have developed protective adap- tations, e.g., of lurking in crevices of rocks or in burrows in sand and lead an alert and watchful, if not a nervous and timid life. This must have Gastric Mill in Deca5oda 421 necessitated a hurried swallowing of food material without its proper masti- cation in the buccal cavity. The presence of the gastric mill in the cardiac stomach, and the consequent absence of the highly specialised mandibles in the buccal cavity is thus an adaptation demanded by a new mode of locomotion and a new environment, by a change in habits and habitats of the animals. 7. Summary. For convenience of treatment the sub-order Macrura has been divided into two groups : (a) Reptantous Macrura comprising crayfishes and lobsters, and (b) Natantous Macrura comprising prawns and shrimps. The present communication deals with the former group characterised by a universal presence of the gastric mill. A large mesocardiac and small pterocardiac ossicles and indistinct exopyloric ossicles bring this group nearer to those Natantous Macrura in which the beginnings of the gastric mill are already found. The lateral accessory teeth may be present or absent. The cardiac pyloric valve is simple and covered with set2e, excepting in some Thalassinidea in which it is chitinised and toothed. The mandibles are remarkably uniform and simple and are adapted for holding, tearing and shovelling morsels of food into the mouth. Each mandible consists of a median cutting plate, the incisor process, a molar prominence and a two- or three-jointed palp. An attempt is made to correlate the presence of the simple mandibles and the complex gastric mill, which indicates that proper mastication of food takes place after it is swallowed, with reference to the fact that (a) the lobsters and the crayfish derivatives of lobsters are derived from prawn-like swimming forms in which the beginnings of gastric mill are found and, therefore, the gastric mill may be regarded as a legacy from these ancestors; (b) a change from a swimming to a crawling or climbing mode of locomotion has resulted in adaptations of habits and habitats more or less similar to those found in Anomura and Brachyura which necessitated transference of mastication from mandibles in the buccal cavity before swallowing to the gastric mill in the cardiac stomach after swallowing.

B2 r 422 S. S. Patwardhan

EXPLANATION OF FIGURES. The ossicles of the gastric mill viewed on the same plane. Inner view. FAG. 1.— 'epprops norvegicus Leach. FIG. 2.—Palinurus vulgaris Latr. Fm. 8.—Callianassa : u^ terranea L-,-^ch. PIG. 4.—Inner view of the " head" of the mandible.

A. N'ephrops norre .7icus Leach. B. Homaru4 vulgaris M. Edw. C. Palinurus vulgaris Latr. I). Scyllarus arctus Fabr. E. Callianassa subterranea Leach. F. Gebia littoralis Desm. REFERENCE LETTERS. c.p.v. .. Cardiac pyloric valve. p. .. Pyloric ossicle. ex.p. .. Exopyloric ossicle. pr.p. .. Prepyloric ossicle. l.a.i. .. Lateral accessory tooth. pt.c. .. Pterocardiac ossicle. l.t. .. Lateral tooth. u.e. .. Urocardiac ossicle. in.c. .. Mesocardi.ac ossicle. z.c. .. Zygocardiac ossicle. m.t. .. Median tooth. REFERENCES Caiman, W. T. .. .. The Life of Crustacea, 1911. Huxley, T. H. .. .. International Scientificc Series, " The Cray Fish," London, 1880, 28. Patwardhan, S. S. .. .. Proc. Innd. Acad. Sci., Bangalore, 1935, 1, Nos. 7 and 8. Powell. A. .. .. .. Lessons in Practical Biology for Indian Studenis, Bombay, 1913. Williamson, H. C. .. .. Report of the Scottish Fishery Board, 1904.