XII 



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 



nepTt 



(Z,7L 



000. Chiton, nephridial and genital systems, an. anus ; cten. ctenidia ; gen. ap genital 

 aperture ; yon. gonad ; gpnod. gonoduct ; mo. mouth ; neph.ap. nephridial aperture-; n. ptr 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 

 t 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 stomodeeum, A 



