CRUSTACEA. 521 



at first simple. The two posterior soon become biramous. The larva leaves 

 the egg before any further appendages become formed. 



Comparative development of the organs. 



Central nervous system. The ventral nerve cord of the 

 Crustacea develops as a thickening of the cpiblast along the 

 median ventral line ; the differentiation of which commences in 

 front, and thence extends backwards. The ventral cord is at 

 first unsegmented. The supra-cesophageal ganglia originate as 

 thickenings of the epiblast of the procephalic lobes. 



The details of the above processes are still in most cases very imper- 

 fectly known. The fullest account we have is that of Rcichenbach (No. 488) 

 for Astacus. He finds that the supra-cesophageal ganglia and ventral cord 

 arise as a continuous formation, and not independently as would seem to be 

 the case in Chatopoda. The supra-cesophageal ganglia are formed from the 

 procephalic lobes. The first trace of them is visible in the form of a pair of 

 pits, one on each side of the middle line. These pits become in the 

 Nauplius stage very deep, and their walls are then continued into two ridges 

 where the epiblast is several cells deep, which pass backwards one on each 

 side of the mouth. The walls of the pits are believed by Reichenbach to 

 give rise to the optic portions of the supra-cesophageal ganglia, and the 

 epiblastic ridges to the remainder of the ganglia and to the circum-cesopha- 

 geal commissures. At a much later stage, when the ambulatory feet have 

 become formed, a median involution of epiblast in front of the mouth and 

 between the two epiblast ridges gives rise to a central part of the supra- 

 cesophageal ganglia. Five elements are thus believed by Reichenbach to be 

 concerned in the formation of these ganglia, viz. two epiblast pits, two 

 epiblast ridges, and an involution of epiblast between the latter. It should 

 be noted however that the fate neither of the pair of pits, nor of the median 

 involution, appears to have been satisfactorily worked out The two 

 epiblast ridges, which pass back from the supra-cesophageal ganglia on 

 each side of the mouth, are continued as a pair of thickenings of the cpiblast 

 along the sides of a median ventral groove. This groove is deep in front 

 and shallows out posteriorly. The thickenings on the sides of this groove 

 no doubt give rise to the lateral halves of the ventral cord, and the cells of 

 the groove itself are believed by Reichenbach, but it appears to me without 

 sufficient evidence, to become invaginated also and to assist in forming the 

 ventral cord. When the ventral cord becomes separated from the epiblast 

 the two halves of it are united in the middle line, but it is markedly bilobed 

 in section. 



In the Isopoda it would appear both from BobrcUky's and Bullar's 

 observations that the ventral nerve cord arises as an unpaired thickening of 

 the epiblast in which there is no trace of anything like a median involution. 

 After this thickening has become separated from the cpiblast a slight 



