COMMON CRAYFISH. 185 



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. It is to be regarded as a cuticular structure 

 forming a storehouse of calcareous matter preliminary to the moult, which is not 

 effected healthily unless the gastroliths are previously and well developed. If they 

 fail to be developed properly, the animal, so it is said, usually dies. 



The cuticular linings of the stomach and intestine are thrown off and regenerated 

 at each moult. The parts of the gastric mill are said to be first of all broken up in 

 all Decapoda. The remaining parts are moulted entire. 



Tubular glands occur in Astacus, as in all Decapoda^ in the walls of the oesophagus 

 and the terminal dilated portion of the intestine. Glands have also been found by 

 Braun on certain of the mouth-appendages, e. g. in Astacus, to the number of twenty 

 on the ' median flattened ' joint of the first maxilla, ' opening on its outer surface,' 

 and on the 'lingula' (=metastoma). They resemble the glands of the branchial 

 cavity (see ante, p. 166) rather than those of the digestive tract. 



The mesenteron (=archenteron), or median portion of the digestive canal, is 

 extremely short as in all higher Crustacea. It has a dorsal caecum, and receives on 

 each side the common liver duct. Its cells form no cuticle. The liver, or hepato- 

 pancreas, derived as an outgrowth from the mesenteron, consists of a right and left 

 gland, each consisting of an anterior, a median, and a posterior lobe. The secretory 

 portion consists of innumerable tubular caeca lined by cells. These cells in all 

 Crustacea possess a fringe of fine hairs affixed to a membrane, which is probably 

 porous. The protoplasm is distinctly striated, especially in Isopoda. In Decapoda 

 there are two kinds of cells, one that secretes coloured drops of fat and contains 

 masses of small globules; the other, fine and coloured granules (= ferment-cells). 

 In the Isopoda there is one kind of cell only forming both fat and fine granules, 

 whilst the globules are absent (Frenzel). There are cells in reserve destined to 

 replace those which are destroyed. The secretion is acid in reaction and con- 

 tains, in many instances, cholesterin, in the Crayfish haematin, as well as a diastatic, 

 peptic, tryptic, and possibly a fat-destroying ferment. Glycogen has been found in 

 the gland (cf. Vitzou, A. Z. Expt. x. p. 554). 



The ' green gland,' or renal organ, opens on the inner side of a ventrally-placed 

 papilla upon the basal joint of the second antenna (p. 169). The duct, lined at its 

 commencement by chitinous cuticle, widens out into a thin walled sac which, to- 

 gether with the gland itself, lies in the thorax at the base of the antenna, the sac 

 dorsally to the gland. The latter forms a disc-like body composed of a tube 

 coiled upon itself and divisible into three sections : (i) a long whitish tube which 

 opens into the sac ; (2) a green-coloured tube opening into (3) a triangular yellow- 

 brown lobe. The coils are so disposed that the third section of the tube lies 

 centrally and dorsally ; the green section forms the outer circumference, and the 

 white section lies between the two others. The bloodvessels are derived from the 

 antennary and sternal arteries, and are especially numerous on the terminal lobe. 

 Nerves derived from the supra- and the infra-oesophageal ganglia are distributed to 

 the excretory sac. This sac and the tube are lined throughout by a single layer of 

 epithelium supported by a fine structureless tunica propria. In the green section 

 the cells have a striated cuticle, and the protoplasm is striated as in the tubuli 



