132 



VELIGER STAGE 



CHAP. 



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

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

 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 the veliger stage, subsequent to the appearance 



TTLg-i^s^g 



Fig. 45. — Veliger oi Patella vulgata 

 L., 180 hours old:/, rudimeutary 

 foot; op, operculum; sh, shell; 

 V, V, velum. (After Patten, 

 highly magnified.) 



Fig. 46. — Developed larva of Cyclas cornea 

 L. : br, rudimentary branchiae ; by, bys- 

 sus;/, foot; ni.e, mantle edge; sh, shell. 

 (After Ziegler, highly magnified.) 



of the velum itself, are the development of the visceral sac and 

 shell-gland on tlie 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 DrPissensia : 

 f, foot; m, mouth; sh. 

 shell ; V, v, velum. 



•-V (After Korschelt and 



Heider, much enlarged.) 

 B, Veliger of a Ptero- 

 pod {Tiedemannia) : op, 

 operculum; s/j, shell; v, 



^ojj 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 disap- 

 pears ; the permanent shell then forms over the surface of the 

 visceral hump fi'om the original centre of the shell-gland. It is 



