132 



VELIGER STAGE 



the latter being situated on the ventral side, between the nioutli 

 and anus, the former on the dorsal side, behind the velum, and 

 above the surface of the eventual visceral sac. Thus the prime 

 characteristics of tlie veliger stage, subsequent to the appearance 



Fig. 45. — Veligm- of PtiteUn vulgata 

 L., 130 liours old :./', nuliiiieutary 

 foot ; o}), operculum ; a7(, shell ; 

 V, V, velum. (After Patten, highly 

 maguitied.) 



Fig. 46. — Develoiicd larva of Ciiclas cornea L. : 

 hr, rudimenlai'v branchiae ; hn, byssus ; /, 

 foot ; m.e, mantle edge ; .s7(, shell. (After 

 Ziegler, highly luaguilied.) 



of the velum itself, are the development of the visceral sa.c and 

 shell -gland on the upper, and of the foot on the under side. 

 According to Lankester the primitive shell-gland does not, as a 



Fig. 47. — A, Advanced 

 veliger of Dreissensia : 

 f, foot ; m, mouth ; ,«/;, 

 .shell; n r, velum. (After 

 Korschelt and Heider, 

 much enlarged.) B, 

 A'cligcr of a Pteropod 

 ( Tiah'mannia) : y/?, oper- 

 culum ; sh, shell ; v, 

 velum. (After Krohn, 

 much enlarged). 



rule, directly give rise to the shell of the adult mollusc, but 

 becomes filled up by a horny substance, and eventually disappears : 

 the permanent shell then forms over the surface of the visceral 

 hump from the original centre of the shell-gland. It is only in 



