207 



itself. Salensky thinks that this middle layer arises from the. 

 outer. It appears first on the ventral side of the embryo. The 

 germ is now of the form indicated by Fig. 136 (ce, ectoderm; 

 'ce, middle layer, the yolk spheres representing the inner layer, 

 endoderm ; m, mouth ; v, velum ; /, foot. After Salensky ).- 



The next important chapter in the life history of the Calyptra?a. 

 is the appearance of the mantle, which arises as a disk-like thick- 

 ening of the outer germ layer on the back of the embryo. In the 

 middle of the disk the shell grows out as a cup-like cavity which 

 is connected only around the edge with the mantle, but is free in 



the eyes, begin as an infolding of the outer germ-layer. 



Up to this time the entire body has been symmetrical. Along 

 the longitudinal axis of the body are the foot, the head-vesicle, 

 the germ of the alimentary canal, and on each side a lobe of the 

 velum. The alimentary canal, now further developed, begins to 

 curve to the left, and as the shell grows, the visceral sack, or post- 

 organs of sense appeared', the ears with their otolites, and the eyes 

 nervous system, and then'it was not the cephalic, but the ganglion 

 a mass of nerve cells from the 



Fig. 137 (after Salensky) re- 

 presents the asymmetrical larva 

 with the shell enveloping a large 

 part of the body, and the velum 

 (v) and foot (/) well developed. 

 The larval head forms a third 

 of the whole body and is still 

 finely ciliated. The temporary 



larval heart (h), a large Oval Veliger of Calyptr^i fhrthcr advanced. 



vesicle, is situated on the right side of the back of the embryo, 



erent position from that of the adult heart, which afterwards arises 



times a minute. This is an entirely different organ, says Salensky, 

 from the pulsating vesicle or "heart" seen by Duben and Koren 

 in Purpura (Fig. 138, P. lapillus and egg capsules, from Verrill's 



