1. The green gland of Palæmonetes (and Palæmon) at the time of hatching of the larva has not developed a lumen, although the external opening can be detected. When the larva leaves the egg the lumen commences to open, and the gland consists of an end-sac and a U-shaped tube, of which the distal portion gives rise to the bladder. The bladder then enlarges greatly, growing at first inwards towards the middle ventral line, then upwards, within the oesophageal nerve-ring and anterior to the oesophagus, to the middle dorsal line, where it meets its fellow of the opposite side. The two bladders grow backwards over the stomach and beneath the dorsal sac, subsequently fusing together in the middle line to form the unpaired nephro-peritoneal sac.

2. The shell-glands are the functional excretory organs at the time of hatching and during the latter part of the embryonal period. They open at the bases of the second maxillæ, and each consists of an end-sac and a Y-shaped renal tube, which have the typical structure of a crustacean nephridium.

3. A dorsal sac, which is completely enclosed by an epithelial lining, persists in adults of Palsem on, Palæmonetes, and Crangon. This sac, which does not contain blood, lies upon the nephro-peritoneal sac and the front end of the ovary, being much enlarged at its posterior end. The cephalic aorta (ophthalmic artery) lies within the dorsal sac.

4. At its anterior end the dorsal sac is surrounded by a mass of tissue which appears to be producing blood-corpuscles.

5. The dorsal sac is formed as a hollowing out in masses of mesoderm-cells, which lie on either side of the cephalic aorta. Two lateral cavities are thus formed, which increase in size and unite below the aorta. Taking into account this mode of development, a comparison with Peripatus shows that the dorsal sac is homologous with the dorsal portions of the mesoblastic somites of that animal, and must therefore be regarded as a true cœlom.

6. The body-cavity of these Crustaceans varies in different regions.

(a) In the anterior part of the thorax it consists of a true cœlom (the dorsal sac and nephridia) and a hæmocœle. The hæmocœle consists of (1) a central cavity, in which the stomach and intestine, the liver and the nerve-cord lie; (2) two lateral cavities, which contain the end-sac and proximal end of the tube of the shell-gland, and which communicate with the central cavity and with the cavities of the legs; and (3) these leg-cavities, which, in the second maxillæ, contain the tube of the shell-gland.

(b) In the posterior part of the thorax the body-cavity is entirely a hæmocœle. It consists of (1) the pericardial cavity, in which lies (2) the heart, and which is separated by the pericardial septum from (3) the central cavity of the body, which contains the genital organs, liver, intestine, and nerve-cord; (4) the lateral cavities, which communicate with the central cavity and with (5) the cavities of the legs.

(c) In the abdomen the body-cavity is entirely a hæmocœle. It consists of a dorsal and a ventral sinus, which communicate with one another by lateral sinuses.

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