412 EDGAR J. ALLEN. 



pied by the dorsal sac — that is to say, the space between the 

 cephalic aorta and the row of corpuscles close to the liver — is 

 absolutely free from corpuscles. 



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

 mass of tissue, which is seen in transverse section in fig. 12 

 (compare also figs. 10, 11, and 15,/. g.). Before commencing 

 this research, Professor Weldon, to whom the existence of this 

 mass of cells was known, and who has indicated it in his figures 

 (No. 20, pi. i, figs. 1 — 3), suggested to me that blood-corpuscles 

 were being budded off by the tissue, and my preparations cer- 

 tainly tend to support this view. In fig. 12 I have endeavoured 

 to represent as nearly as possible the appearance presented by 

 a section, and it is difficult to account for it in any other way 

 than that which Professor Weldon suggests, since the outlying 

 cells cannot be distinguished from blood-corpuscles found in 

 other parts of the body. 



A pair of muscles must also be mentioned, which run on 

 either side of the cephalic aorta in the anterior portion of the 

 dorsal sac. These muscles (figs. 11, 12, 15, c. m.) run, in the 

 adult, from the median dorsal surface of the carapace to points 

 just below the angle made by the inferior surface of the rostrum 

 with the front end of the head. What their function is in 

 Palaemonetes is not apparent, but they would seem to be 

 homologous with the muscles which move the rostrum in other 

 forms. 



Development of Dorsal Sac. — According to Reichenbach 

 for Astacus (No. 18), and Kingsley for Crangon (No. 13), 

 the cephalic aorta of Decapods is formed from mesoderm-cells 

 which have broken away from the ventral bands and wandered 

 to the dorsal surface, where they unite to form a tube. Al- 

 though I have not specially studied these stages, I may say 

 that one series of sections which I have made tends to confirm 

 the view that these cells travel up on either side between the 

 liver lobes and the ectoderm ; and the mode of development of 

 the heart of Brancliipus, as described by Claus (No. 6), in 

 which animal solid masses of mesoderm grow up on both 



