XII 



PHYLUM MOLLUSC A 



669 



mo 



den 



nepTt 



products are mature. Each is an unpaired sac marked by a series 

 of slight lateral constrictions. 



Little is known of the development of the Aplacophora. The 

 eggs undergo complete segmentation, and give rise to a gastrula 

 by imagination. This develops into a form of trochosphere 

 with a ciliated ring called the prototroch or velum. 



The eggs of Chiton are fertilised in the mantle-cavity, where in 

 one species they are 

 retained until the 

 embryos are fully de- 

 veloped. At first the 

 segmentation is toler- 

 ably equal the ovum 

 becoming divided into 

 four approximately 

 equal blastomeres ; 

 but at the stage of 

 eight cells four on 

 one side are to be dis- 

 tinguished as larger 

 than the other four. 

 These two sets un- 

 dergo further divi- 

 sions, and arrange 

 themselves in such 

 a way as to form a 

 somewhat flattened 

 blastula, one side of 

 which (vegetal pole) 

 is composed of a com- 

 paratively small num- 

 ber of large cells. 

 Then follows the in- 

 vagination of the 

 cells of the vegetal 

 side and the resulting 

 formation of a gast- 

 rula. This soon be- 

 comes 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. 570, B, mes.) ; these pass into the segmen- 

 tation-cavity, and speedily assume a bilateral arrangement. 



Two rings of cells surrounding the embryo develop cilia (cil.\ 

 and by the double circlet thus formed the larva becomes divided 

 into an anterior and a posterior region. The blastopore becomes 





n.pen.ctp 

 pTi.ctp 



cin. 



FIG. >t'>9. Chiton, nephridial and genital systems, an. 

 anus; etc a. ctenidia ; <! /<. /<. genital aperture ; tjon. gonad ; 

 gonod. gonoduct ; mo. mouth ; nc/>li.n/>. nephridial aperture ; 

 n. peri. ftj). aperture from nephridia to pericardium. (From 

 Simroth, after Haller and Lang.) 



