DEVELOPMENT OP THE EXTERNAL FORM HETEROPODA. 155 



77. 



These specially marked cells lay originally in the ventral middle line ; 

 they however shift towards the right side in consequence of the un- 

 equal growth which takes place also among the Heteropoda, and the 

 anus is thus found on the right 

 side, as we have already seen 

 to be the case in various other 

 Gastropods (p. 142). At this 

 stage, the embryo is almost in 

 the condition of the Trocho- 

 plwre. It then soon passes 

 over to the Veliger stage, the 

 velum being bilobed (Fig. 66). 

 This bilobed character is at first 



made evident by the mouth 



PIG. 66. Larva of Firoloida with velum ex- 

 shifting into a notch of the tended (after KROHN). /, foot ; /, rudi- 



v>vo al nil i* of or! vino- ment of nn ' s * she11 > * ri g ht tentacle, at 



the base of which the right eye is visible ; 

 the left tentacle is still wanting, but the eye 

 is present ; v, velum. 



of de- 



So far, the course 

 velopment in the various 

 Heteropoda seems to be very similar (FOL). 



The round embryo, 



which is now provided with a bilobed velum, a foot and a cup-shaped 

 shell, moves about by means 

 of its cilia within the gela- 

 tinous egg-rope, which has 

 become hollow ; it, however, 

 soon leaves this to swim about 

 as a free larva (Fig. 66), 

 circling slowly in the water 

 (GEGENBAUR). The move- 

 ments become more rapid and 

 the larva more active when 

 the lobes of the velum increase 

 in size and are able to act 

 independently of one another. 

 According to KROHN, in 

 Firoloida and Phrotrachea, 

 the velum becomes drawn out 

 on each side into two long 



FIG. 67. Larva of Atlanta with extended 

 velum (after GEGENBAUR). /, foot; fl, 

 rudiment of the fin ; op, operculum ; ot, 

 otocyst ; s, shell ; v, velum. 



and very narrow streamers, 



the larva then presenting an 



appearance similar to that of the Velif/er larva depicted in Fig. 54, 



p. 130. In Atlanta, the velum is drawn out into three streamers 



