340 Mr. E. J. Allen. On the Nephridia and Body [Dec. 8, 



lateral cavities, separated from the central cavities by masses of muscle 

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

 sideration, 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 serraius, 

 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. Anteriorly 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 Palcemonetes. 



The dorsal sac does not contain blood. I have been led to this con- 

 clusion for the following reasons : (1.) In a large number of series 

 of sections, both of larvae 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 pre- 

 served specimens it contains a clot, which can generally be dis- 

 tinguished from the surrounding blood clot. (3.) I have observed 

 carefully, and for a long time, living larvse, 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 in- 

 dicated it in his figures, suggested to me this view of its nature, and 

 it is fully supported by my preparations. 



Development of the Dorsal Sac. In embryos of Palcemonetes in which 

 the cephalic aorta is already formed, the cells surrounding the latter 

 are arranged in two layers, an internal and an external. Before the time 

 of hatching arrives, the cells of the external layer enlarge consider- 

 ably, and give rise to the appearance of a solid mass of cells upon 

 either side of the aorta. The dorsal sac is formed by the hollowing 

 oat of these masses of cells. Two lateral cavities are thus formed, 

 which are separated by the aorta. The protoplasm of the cells lining 

 these cavities, which is at first gathered into masses around the 

 nuclei, then spreads out into a thin sheet, drawing away from the 

 lower portion of the aorta, and causing the two lateral cavities to 

 unite ventrally, and so form a single sac. In the region of the first 





