IX MOLLUSCA 311 



which span the blastocoele, extending from gut to ectoderm. In 

 A _ „ B 



ph- ptr-y. 



Fig. 245. — Formation of the coelom iu Paludina mvipara. (After Brlanger. ) 

 A, optical frontal section of embryo in the stage when the coelom is being formed. B, sagittal 



section of embryo in this stage. 

 'p.tr, prototrochal cells. 



the hinder end of each streak 

 there is, however, a compact 

 mass which becomes hollowed 

 out to form a coelomic vesicle, 

 the rudiment of one of the 

 pericardial sacs. The two meso- 

 dermic bands then fuse together 

 in the middle line behind, and 

 the pericardial sacs become 

 pressed against one another so 

 that their conjoined walls form 

 a septum {sept, Fig. 247, B). 



Tonniges denies point-blank 

 the existence of this ventral 

 sac. According to him the 

 adult mesoderm arises as an 

 ectodermal proliferation, on 

 each side of the middle line, 

 which gives rise to the 

 irregular mass of cells seen 



0, transverse section in this stage, co^, coelomic poucli ; (7, gut ; 



Fig. 246. — Optical frontal section of an embryo 

 o{ Paludina vivipara a. little older than those 

 represented in figure 245. (After Erlanger.) 



l.n, larval kidney ; m.&, mesoblastic band ; 

 p.tr, prototrochal cells. 



by Erlanger. Now, we may quite 



