132 



VELIGER SrAGE 



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 



Fig. 45. — Veliger of Patella vulgata 

 L., 180 hours old:/, rudimentary- 

 foot; op, operculum; sh, shell; 

 V, V, velum. (After Patten, 

 highly magnified.) 



Fig. 46. — Developed larva of C'yclas cornea 

 L. : hr, rudimentary branchiae; hy, bys- 

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

 (After Ziegler, highly magnified.) 



of the velum itself, are the development of the visceral sac 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; sh. 

 shell; V, v, velum. 

 (After Korschelt and 

 Heider, much enlarged.) 

 B, Veliger of a Ptero- 

 pod {Tiedemannia) : op, 

 operculum; s/j, shell; i>, 

 velum. (After Krohu, 

 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 



