406 EDGAR J. ALLEN. 



(Esophagus, and between the two nerve-commissures. In the 

 stage figured, it has reached a point which is at a distance 

 from the ventral surface about a quarter as great as the height 

 of the body in this region. 



In a young Palsemonetes^ which has apparently just 

 assumed adult characters^ the enlarged portions of the bladders 

 of the two green glands are seen in sections (fig. 5, bl.) to lie 

 side by side above the stomach in the position occupied by the 

 single unpaired sac of the adult. The latter sac is formed by 

 the fusion of the enlargements of the bladders of the two sides, 

 and in the series from which fig. 5 was drawn this fusion had 

 already commenced in the most posterior portions of the two 

 enlargements. 



The mode of development of this nephro-peritoneal sac 

 (" vessie sus-stomacale impaire'^ — Marchal) proves it to 

 be in reality derived from the fusion of the greatly enlarged 

 bladders of opposite sides, a conclusion which has been already 

 arrived at by Weldon (No. 20) and Marchal (No. 17) from 

 a comparative study of the renal organs of the Decapods. 



With regard to the plexus of tubules found in the adult 

 between the end-sac and the bladder, I can only say that it is 

 derived from the single short renal tube which in the larva 

 connects the end-sac with the bladder ; that is to say, the 

 portion between the opening of the end-sac and the point in- 

 dicated by the letter E in fig. 1. This tube enlarges con- 

 siderably in size, but remains single until a late stage of larval 

 life ; whilst in the youngest adult of which I possess sections 

 there are already several smaller tubules, which have ob- 

 viously been derived by the splitting up of the single tube, 

 but exactly how the process has taken place I do not know. 

 "What has in all probability happened is simply that opposite 

 walls of the large tube have grown together along certain 

 lines, and thus given rise to the complicated plexus of smaller 

 ones. 



The Shell-gland. — As long ago as 1876, Claus (No. 2) 

 predicted that a rudiment of the shell-gland would in all 



