ON THE STRUCTURE AND MECHANISM OF THE GASTRIC MILL IN DECAPODA. 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 crayfishes, lobsters and allied forms and characterised by possessing a dorsoventrally flattened body and a crawling and climbing mode of locomotion, and (2) Natantous Macrura comprising prawns and shrimps, 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 crayfish, 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 species 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 Scyllarus 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 animal 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.