Claus has pointed out (Arb. Zool. Inst. Wien, vi. 1885, pp. 39-47) (1) that the relation of the coxopodite to the body-walls is by no means a constant one in Crustacea, and the arthrodial membrane has limits often ill-defined; (2) that in the branchiferous Schizofioda (Eufhausia, etc.) the branchiae, which are all trifid, form a row of podobranchiae; (3) that in the larva of Penaeus there are three rows of branchial rudiments, which he terms distal, middle and proximal. The last is a double row, but one of the elements appears at a later period than the other. The distal rudiment on each limb represents the podobranch + epipodite(=lamina): the middle and the first proximal, the anterior and posterior arthrobranchs, and the second proximal the pleurobranch, in Astacus. It may be noted that the primitive distal rudiment from the second maxilliped to the third thoracic foot inclusive pushes out a basal bud. The bud becomes a branchia, afterwards lost on all the feet save the second maxilliped, while the primitive rudiment forms an epipodite.
The three sets of rudiments which first appear belong distinctly to the original basal joint of the limb, but the arthrodial membrane develops in such a manner that they come to lie subsequently on the coxopodite, the arthrodial membrane, and the epimera.
The cavity of the branchial stem is divided by a septum into an outer and inner channel communicating at the apex. The latter is continuous with one of the six blood passages or 'branchial veins' which open into the pericardial sinus with widened mouths. The former is continuous with bloodvessels coming from the sternal sinus lodged in the sternal canal. Each branchial filament is similarly divided by a septum incomplete at the apex. According to Haeckel the blood-spaces are intercellular spaces, or lacunae, and not true vessels. The tissue is spongy. The cells are pyriform, one end pointed and united to the cuticula, the other swollen and nucleated, and attached to other cells (cf. Haeckel, Arch. Anat. u. Phys. 1857, p. 554).
The oesophagus and stomach (=stomodaeum), and the intestine (=procto-daeum) are lined by a chitinous coat. This coat consists of a superficial delicate cuticle similar to that of the carapace, and a deeper lamellate layer, sometimes penetrated by pores, especially where it is much thickened. There are numerous setae in the stomach, principally in its pyloric portion, and in the intestine. They are of two kinds, hollow hairs, similar to those of the carapace, very plentiful in the pyloric portion, and solid continuous processes. Minute ridges secreted by the chitino-genous cells, and corresponding three or more to a single cell, occur in the intestine where the chitinoid coat is divided into areae corresponding, as on the carapace, to the individual cells. The cuticular coat of the stomach ends at the pylorus with five projecting processes; that of the intestine commences with six elevations prolonged into ridges which traverse the tube in a spiral fashion. Beneath the chitinoid coat is the single layer of chitinogenous, or ectoderm cells, large in size, and then a fibrous membrane followed by a layer of cellular connective tissue which incloses muscle fibres, both longitudinal and circular, in the stomach and intestine.
The chitinoid layer of the stomach is thickened in the dorsal and lateral walls to form certain ossicles which, according to Vitzou, have the same structure as the carapace. Some of these ossicles have simply a supporting function, others constitute the 'gastric mill.' The latter are named and arranged as follows. There is a cardiac ossicle crossing the cardiac region transversely, articulating laterally with a ptero-cardiac piece, and extending forwards into a softer disc upon which the anterior gastric muscles are principally inserted, and backwards into a narrow uro-cardiac piece which is produced inferiorly into two accessory or cardiac teeth, rudimentary in the Lobster. A pyloric ossicle crosses the pyloric region transversely and dorsally, and gives insertion to the posterior gastric muscles. It articulates in front with a prepyloric ossicle which is bent downwards so as to form with it an acute angle backwards way, and articulates in turn with the urocardiac piece (supra). Close to this articulation the prepyloric ossicle is produced into a bifid median tooth, single in the Lobster. The pyloric ossicle articulates laterally on each side with a zygocardiac ossicle lying in the walls of the cardiac region.
This ossicle articulates at its outer extremity with the corresponding extremity of the ptero-cardiac ossicle (supra) of its own side. Its inner extremity bears the great serrated lateral tooth. Just below the anterior end of this tooth projects an infero-lateral tooth borne by the 'lateral cardiac piece' of Milne-Edwards, one of the supporting bars of the stomachal walls. The stomach possesses extrinsic and intrinsic muscles. The former set includes the gastric muscles above mentioned, as well as the anterior lateral, the posterior, superior and inferior dilators. The latter set includes various muscles. One system of fibres unites the ptero- and zygo-cardiac ossicles. The remainder, according to Mr. T. J. Parker, act as constrictors, especially a layer which embraces the pyloric region. This region has its cavity, more particularly in the posterior part, narrowed by the bulging inward of its side-walls and the development of a median ventral ridge. The surfaces of these parts are beset with setae and form a most efficient 'filter.'
There is on either side of the stomach, at the entrance of the oesophagus, a round white spot caused by the presence of a flattish papilla having the same structure as the rest of the wall of the stomach. Forty days before a moult in the adult, or for a shorter period in the young according to age, the chitinogenous cells of this papilla develope a number of minute knobbed processes which raise the overlying chitinous cuticle and eventually break up into corpuscle-like bodies. Beneath the cuticle, and between the ends of the chitinogenous cells and their processes, lamellae of calcified organic matter are laid down forming the gastrolith. The lamellae are pierced by pores. Their substance consists of Calcium carbonate, with a small admixture of phosphate, and of organic substances partly soluble in water, partly insoluble, and perhaps chitinoid in nature. During the development of the gastrolith the papilla becomes more prominent and changes its shape. The fully formed stone is slightly concave and smooth on its stomachal or inner face, convex and marked by ridges on its external face. The lamellae of this face are the last formed and the hardest. The stone is eventually cast off into the stomach, previous to the moult, and ground down.