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 contains, 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, together 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: (1) 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 contorti of the Mammalian kidney.

Guanin and uric acid have been stated to occur in the gland. Grobben compares the terminal lobe with its rich vascular supply to the Malpighian capsule; the remaining sections to the tubuli uriniferi of the Vertebrate kidney. The shell gland of the Phyllopoda and Copepoda consists of a similar terminal lobe and tube, but it opens on or close to the second maxilla.

The ovary is a trilobed gland like the testis, but the fissures between the lobes are not so deep. The oviducts originate as do the vasa deferentia, but they are wide, short, and straight. The ovary is lined by a delicate cuticle. The ovum is developed by the growth of a single cell out of a small mass of cells. It has a vitelline membrane, and when ripe, is set free into the cavity of the ovary. It is fecundated externally to the body, and is suspended to the feet, sterna, etc, of the abdomen during the development of the embryo. For the mode of suspension and the cement glands, see ante, p. 166.

The testis is a tubular gland. The tubes branch, and the ultimate branches end in a number of short stalked vesicles varying in size according to the state of their contents. The lining cells multiply and are differentiated into spermatozoa. These bodies are disc-like structures with a number of slender curved rays attached to the circumference of the disc. The structure of the disc is still somewhat obscure. The spermatic fluid is milky, and contains a viscid substance which agglutinates the spermatozoa into thread-like spermatophores. The sperm is shed through the channels of the first pair of abdominal limbs. The second pair are worked to and fro in these channels as if to keep them clear. The male throws the female on her back and deposits the sperm on the ventral surface of the last pair of swimmerets and on the thoracic sterna round the oviducal apertures, parts which are approximated during oviposition.

Branchiae.Huxley, P. Z. S. .1878; Claus, Arb. Zool. Inst. Wien, vi. 1885.

Digestive tract; structure of walls, glands, and epithelium. Vitzou, A. Z. Expt. x. 1882; Braun, Arb. Zool. Zoot. Inst. Wurzburg, ii. 1875, iii. 1876-7; Frenzel, A. M. A. xxv. 1885. Ossicles of stomach and muscles. Mocquard, A. Sc. N. (6) xvi. 1883; T. J. Parker, Journal Anat. Phys. 1877; Albert, Z. W. Z. xxxix. 1883. Working model of gastric mill. Roth, Nature, xxi. 1879-80. Pyloric filter. Huxley, The Crayfish, p. 58. Gastrolith. Braun, Arb. Zool. Zoot. Inst. Wurzburg, ii. 1875; Chantran, C. R. lxxviii. and lxxix. 1874; its chemical composition, Dulk, Muller's Archiv (Arch. f. Anat. und Phys.), 1835.

Structure of liver.Frenzel, Mitth. Zool. Stat. Naples, v. 1884. Its Chemical action. - Krukenberg, Untersuch. Phys. Inst. Heidelberg, ii. 1882. Its Colouring matter. MacMunn, P. R. S. xxxv. 1883.

Green gland.Wassiliew, Z. A. i. 1878; Grobben, Arb. Zool. Inst. Wien, Hi. 1881. For guanin and uric acid in it, cf. Griffiths, P. R. S. xxxviii. 1885.

Testis.Lemoine, A. Sc. N. (5) x. 1868; Rougemont, Organes genitaux, etc, Astacus, Bull. Soc. Sc. Nat. Neuchatel, ii. 1880 (not seen). Spermatozoa. Grobben, Arb. Zool. Inst. Wien, i. 1878; cf. note in Huxley, The Crayfish, p. 354. Spermatogenesis. Sabatier, C. R. c. 1885; Hermann, C. R. xcvii. 1883.

Fecundation.Chantran, C. R. lxxi. 1870; lxxiv. 1872.

Structure of ovum.Waldeyer, Eierstock und Ei, Leipzig, 1870, p. 85; cf. general account by Ludwig, Arb. Zool. Zoot. Inst. Wurzburg, i. 1874.

Cement glands.Braun, Arb. Zool. Zoot. Inst. Wurzburg, ii. 1875; iii. 1876-7. Their secretion and fixation of ova. Lereboullet, A. Sc. N. (4), xiv. i860.