304 ANIMAL BIOLOGY. [Part II. 



In Fig. 88, D., the embryo is shown at a later stage, known as 

 the veliger stage. The incomplete circlet of cilia has become the 

 velum (v.) in front of the mouth (m.). The foot has become 

 bi-lobed, and the collar is developed. The visceral hump is 

 covered with a delicate shell. At t. the tentacle is being 

 developed. 



The common snail does not seem to pass through a trocho- 

 sphere arid veliger phase, these stages in molluscan existence 

 being shortened or suppressed. 



Mesoblast takes its origin at the lips of the blastopore. 



The nerve-collar arises as several epiblastic thickenings. 



