THE DEVELOPMENT OF A MOLLUSK 49 



the circulatory system, heart and blood-vessels and the support- 

 ing tissues of the body. 



Coincident with this differentiation of the regions of the body 

 into organs, a change in the direction of the antero-posterior 

 axis of the embryo takes place. The whole posterior portion is 



MODEL 20 



The formation of the vehger larva has proceeded farther. The various 

 external organs are well defined. Below is seen the shell secreted by the under- 

 lying cells of the shell gland. 



pushed ventrally : the mouth opening and the whole apical pole 

 are shifted forward, and there is a twisting of the entire axis, 

 plainly seen in the bending of the intestine. This organ, which 

 originally lay in the mid-ventral line, assumes the form of an 

 almost complete loop (Model 22). The asymmetry of the 



