TROCHOSPHERE AND VELIGER STAGES 



131 



tion of a circlet of praeoral cilia, dividing the still nearly spherical 

 eniliryo into two unequal portions, tlie smaller of which consists 

 simply of the prostomium, or part in front of the mouth, the 

 larger bearing the mouth and anus. 



So far the series of changes undergone by the embryo are not 

 peculiar to the Mollusca ; we now come to tliose which are 

 definitely characteristic of that group. The stage next succeed- 

 ing the development of the trochosphere is the definitive forma- 

 tion of the velum, a process especially characteristic of the 

 Gasteropoda and I'elecypoda, but apparently not occurring in the 

 great majority of land Pulmonata. 



The circlet of cilia becomes puslied more and more towards 



Fig. 44. — Veligers of Dentaliuvi entalis L.: A, longitudinal section of a larva 14 hours 

 old, X 285 ; B, larva of 37 hours, x 165 ; C, longitudinal section of larva of 34 

 hours, X 165 ; m, mouth ; v v, velum. (After Kowalewsky). 



the anterior portion of the embryo, the cilia themselves become 

 longer, while the portion of the body from which they spring 

 becomes elevated into a ridge or ring, which, as a rule, develops 

 on each side a more or less pronounced lolje. The name velum 

 is applied to this entire process of ciliated ring and lobes, and 

 to the area which they enclose. 



In this so-called vcliger stage, the velum serves, in the first 

 place, to cause rotation of the larva within the egg-capsules, and, 

 after hatcliing, as an organ of locomotion. As a rule, the velum 

 disappears entirely in the adult mollusc after the free-swimming 

 stage is over, but in the connnon Limnaea stagnalis it persists, 

 losing its cilia, as the very prominent circum-oral lobes. Simul- 

 taneously with the -development of the velum, and in some cases 

 earlier, appear the rudiments of the shell-gland and of the foot. 



