TROCHOSPIIERE AND VELIGER STAGES 



131 



dactive systems. The next, or trocho sphere (trochophora) stage, 

 involves the formation of a circlet of praeoral cilia, dividing the 

 still nearly spherical embryo into two unequal portions, the 

 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 those w^hich are 

 definitely characteristic of that group. The stage next succeed- 

 ing the development of the trochosphere is the definitive for- 

 mation of the velum, a process especially characteristic of the 

 Gasteropoda and Pelecypoda, but apparently not occurring in 

 the great majority of land Pulmonata. 



Fig. 44. — Veligers of Dentalium entails 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 circlet of cilia becomes pushed more and more towards 

 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 lobe. The name vehwi 

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

 to the area which they enclose. 



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

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

 after hatching, as an organ of locomotion. Asa rule, the velum 

 disappears entirely in the adult mollusc after the free-swimming 

 stage is over, but in the common 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 



