THE VELIGER LARVA. 



131 



velum, the dorsal union of the two ridges often taking place very 

 late. In its later development, in the Prosobranchia and especially in 

 the Opisthobranchia, Heteropoda, and Pteropoda, the velum by its 

 great lateral growth, assumes a bilobed form (Fig. 55 A-C). It 

 becomes at the same time very large and is a most efficient locomotory 

 organ. It is beset with large, strong cilia, which may be replaced by 

 much smaller cilia at the junction of the two wing-like lobes, the bila- 





7 \W'</> \ \ r-'-"- ^■■■'■■■r\ 



PlG. 55. — A, embryo, B and C, Veliger larvae of Vermetus at different stages (after 

 La.caze-Duthikrs). A, dorsal aspect; B, ventral aspect; C, lateral aspect, a, 

 eyes ; c, rudiments of the cerebral ganglia ; /, foot ; m, mouth ; ot, otocyst ; <y>, 

 operculum ; s, shell, t, tentacle ; v, velum. 



teral character thus becoming still more apparent (Fig. 72, p. 162). The 

 larval stage which is provided with this very characteristic locomotory 

 apparatus has been called the Veliger stage (Kay Lankester). The 

 great size which may be attained by the velum can be seen from 

 Fig. 54, which represents the Veliger larva of a Prosobranch (species 



