DEVELOPMENT OK THE EXTERNAL FORM HETEROPODA. 



155 



FlG. 66. — Larva of Firoloida with velum ex- 

 tended (after KROHN). ,/', foot; fl, rudi- 

 ment of fin ; .s\ shell ; /, right tentacle, at 

 the base of which the right eye is visible ; 

 the left tentacle is still wanting, but the eye 

 is present ; r, velum. 



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- 

 phore. 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 

 shifting into a notch of the 

 pre-oral ciliated ring. 



So far, the course of de- 

 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 PterotracTiea, 

 the velum becomes drawn out 

 on each side into two l">ng 

 and very narrow streamers, 

 the larva then presenting an 

 appearance similar to that of the Veliger larva depicted in Fig. 54, 

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



Fig. 67. — Larva of Atlanta with extended 

 velum (after Gegenbaur). ./', foot; H, 

 rudiment of the fin ; op, operculum ; ot, 

 otocyst ; s, shell ; v, velum. 



