THE GASTROPODA 



139 



Post-larval metamorphoses occur in various cases. The velum, 

 as seen above, disappears, being absorbed by a process of phago- 

 cytosis. In various genera the operculum falls off, and so also does 

 the shell in naked forms (Fig. 116, B) and in Lamellaria, in which a 

 new shell is formed replacing the larval shell or Echinospira. 



It is only in rare cases that a second larval form exists after the 

 disappearance of the velum and before the adult state is reached ; 

 this is the case, however, in the gymnosomatous " Pteropods," in 

 which three transverse and parallel ciliated rings are formed before 

 the fins are completely developed (Fig. 120). The most anterior of 

 these rings is made up of inter- 

 rupted portions ; the two others, 

 on the contrary, are continuous, 

 and are situated respectively at 

 the middle of the body and near 

 the aboral extremity. These two 

 continuous ciliated circles, and 

 especially the more posterior, are 

 preserved till a very late period, 

 and sometimes persist in the adult, 

 whose habits do not differ from 



c. 



those of the larva (Fig. 155). 



Sooner or later after their ex- 

 pulsion from the oviduct, or after 

 the nidus is laid, the eggs are 

 hatched out : after a period of some 

 twenty hours in Trochus, after ten 

 days in certain Nudibranchs (Ter- 

 gipes), at the end of eighteen days 

 in others (Cenia), after three or four 

 weeks in Limnaea-, after more than 

 a month in Valvata and certain 



crta/>iae nf Arinn anrl Tvmnr Tr is Larva of SpowiiobranclMea australis, 



species ot Anon and Umax, it is V(<nlral iisi)ect \- lllollth . CMl< anterjo ; 



Only in exceptional cases that the ciliated ring; c.c.m, middle ciliated rinn; 

 i , i_ i .,, ,-, c.c.p, posterior ciliatfil riiif; ; ?t, fin; yi, 



young are hatched out with the foot. (After Racovitza.) 

 characters of the adult, but this 



is the case in all the Pulmonates with the exception of 

 the Siphonariidae which have a marine veliger larva in the 

 Opisthobranchs Cenia and Euncina, and in sundry Streptoneura 

 such as Littorina and Lacuna among the Taenioglossa, and Piirpum 

 and Buccinum among the Rachiglossa. In normal cases the young 

 Gastropods are hatched out as free-swimming or pelagic veliger 

 larvae (Fig. 61). This veliger has a very small foot and a more or 

 less voluminous velum, the latter organ being smallest in the least 

 specialised forms, such as Trochus (Fig. Ill), Patella, Fissurella, etc., 

 and is the characteristic larval form in most opisthobranchiate 



Flu. 1-20. 



