422 EDGAR J. ALLEN. 



■where it meets its fellow of the opposite side. The two 

 bladders grow backwards over the stomach aud beneath the 

 dorsal sac, subsequently fusing together iu 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 

 maxillae, and each consists of an end-sac aud a Y-shaped renal 

 tube, which have the typical structure of a crustacean 

 nephridium. 



3. A dorsal sac, which is completely enclosed by an epi- 

 thelial lining, persists in adults ofPalaemon, Palsemonetes, 

 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 suiTOunded 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 meso- 

 blastic somites of that animal, and must therefore be regarded 

 as a true coelom. 



6. The body-cavity of these Crustaceans varies iu different 

 regions. 



(a) In the anterior part of the thorax it consists of a true 

 coelom (the dorsal sac and nephridia) and a hsemocoele. The 

 hsemocoele 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 maxillae, contain the tube of 

 the shell-gland. 



