238 Mr. E. J. Allen on the Nephridia and 



larva which is a few days old shows that the cavity enclosed 

 by the external chitin and ectoderm may be divided into four 

 regions: — a dorsal sac, sm'rounded by a definite layer of 

 epithelium and within which the cephalic aorta lies, but which 

 does not itself contain blood ; a central cavity, in which the 

 liver, intestine, and nerve-cord are found ; two lateral cavities, 

 separated from the central cavities by masses of muscle and 

 bands of connective tissue, and which, in the region under 

 consideration, contain the proximal ends of the shell-glands ; 

 and, fourthly, the cavities of the limbs, which contain the 

 distal ends of the same organs. The cavities of the limbs 

 communicate with the lateral cavities, and the latter frequently 

 communicate with the central cavity by the disappearance of 

 the connective-tissue bands. The central cavity, the lateral 

 cavities, and the cavities of the limbs all contain blood. 



The Dorsal Sac. — I have found the dorsal sac in Palcemon 

 serratus, Palcemonetes varians, and Crangon vulgaris, and in 

 the adult it attains a considerable size. If a dissection be 

 made of an adult Palcemon the sac is readily seen. Ante- 

 riorly it appears as an elongated cylindrical tube lying upon 

 the nephroperitoneal sac, and containing within it the cephalic 

 aorta. Posteriorly it is very much enlarged, covering the 

 front part of the ovaries, and running downwards on either 

 side into the cavity which surrounds the intestine and liver. 

 A similar condition of things is found in the adult Palce- 

 monetes. 



The dorsal sac does not contain blood. I have been led 

 to this conclusion for the following reasons : — (1) In a large 

 number of series of sections, both of larvee and adults, I have 

 never seen a blood-corpuscle within the sac. (2) The sac is 

 completely closed and has no communication with the blood- 

 sinuses of the body ; in preserved specimens it contains a clot, 

 which can generally be distinguished from the surrounding 

 blood-clot. (3) I have observed carefully and for a long time 

 living larvffi, and the space occupied by the sac has always 

 been perfectly free from blood- corpuscles. 



At its anterior end the dorsal sac is surrounded by a mass 

 of tissue, from the external surface of which blood-corpuscles 

 appear to be budding off. Before commencing this research 

 Professor Weldon, to whom the existence of this tissue was 

 known, and who has indicated it in his figures, suggested to 

 me this view of its nature, and it is fully supported by ray 

 preparations. 



Development of the Dorsal Sac. — In embryos of Palcemonetes 

 in which the cephalic aorta is already formed the cells sur- 



