CRUSTACEA. 



527 



senteron. The stomachic end of the stomodaeum lies in contact with the 

 mesenteron close to the point where it is continued into the hepatic 

 diverticula, and, though the partition-wall between the two becomes early 

 very thin, a free communication is not established till the yolk has been 

 completely absorbed. 



The alimentary tract in the Isopoda is mainly if not entirely formed 

 from the proctodaeum and stomodaeum, both of which arise before any other 

 part of the alimentary system as epiblastic invaginations, and gradually 

 grow inwards (fig. 244). In Oniscus the liver is formed as two discs 

 at the surface of the yolk on each side of the anterior part of the body. 

 Their walls are composed of cubical cells derived from the yolk cells, the 



FlG. 244. TWO LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF ONISCUS 



MURARIUS. (After Bobretzky.) 



st. stomodreum ; /;-. proctodaeum ; hy. hypoblast formed of large nucleated cells 

 imbedded in yolk ; m. mesoblast ; vg. ventral nerve cord ; sg. supra-cesophageal gan- 

 glion ; //. liver ; do. dorsal organ ; zp. rudiment of masticatory apparatus. 



origin of which was spoken of on p. 516. These two discs gradually take 

 the form of sacks (fig. 244 B, //.) freely open on their inner side to the 

 yolk. As these sacks continue to grow the stomodaeum and proctodaeum 

 uo not remain passive. The stomodaeum, which gives rise to the oesophagus 

 and stomach of the adult, soon exhibits a posterior dilatation destined to 

 become the stomach, on the dorsal wall of which a well-marked prominence 

 the earliest trace of the future armature is soon formed (fig. 244 B, 

 zp). The proctodaeum (pr) grows with much greater rapidity than the 

 stomodasum, and its end adjoining the yolk becomes extremely thin or even 

 broken through. In the earliest stages it was surrounded by the yolk cells, 

 but in its later growth the yolk cells become gradually reduced in number 

 and appear to recede before it so much so that one is led to conclude 

 that the later growth of the proctodaeum takes place at the expense of the 

 yolk cells. 



The liver sacks become filled with a granular material without a trace 

 of cells ; their posterior wall is continuous with the yolk cells, and their 

 anterior lies close behind the stomach. The proctodaeum continually 

 grows forwards till it approaches close to the stomodaeum, and the two 



