PHYLUM MOLLUSCA 



719 



a somewhat flattened blastula, one side of which (vegetal pole) 

 is composed of a comparatively small number of large cells. 

 Then follows the invagination of the cells of the vegetal side 

 and the resulting formation of a gastrula : this soon becomes 

 elongated in the direction of the future long axis. Two 

 endoderm cells of specially large size in the neighbourhood 

 of the blastopore, with several others in their proximity, constitute 

 the rudiments of the mesoderm (Fig.- 607, B, mes.) ; these pass 



den 



nepJt 



ffon 



,ffen..ap 

 ■peri.ttp 

 neph.ap 



czn 



606.— Chiton, neplirjdial and genitaPsystems. aji. anus ; c^ew. ctcnidia^ ^/e-rt. aj5 genital 

 aperture ; (jon. gonad ; gonod. gonoduct ; mo. mouth ; neph,ap. nepliridial aperture ; n, per ap. 

 aperture from nephridia to pericardium. (From Simroth, after Haller and Lang. 



into the segmentation-cavity and speedily assume a bilateral 

 arrangement. 



Two rings of cells surrounding the embryo develop cilia (cil.), and 

 owing to the double circlet thus formed an anterior and a posterior 

 region are distinguishable in the larva. The blastopore becomes 

 shifted from its original posterior position forwards on the ventral 

 surface until it comes to be situated just behind the circlet of 

 cilia ; it undergoes elongation, and an invagination of ectoderm 

 round its anterior end forms the mouth (mo.) and stomodaeum, A 



