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 
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 7rochus, 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 Vualvata and certain ae 
species of Arion and Limaz. It is Larva of Spongiobranchaea «australis, 
x i ventral aspect. 0, mouth; e.c.¢, anterior 
only in exceptional cases that the ciliated ring; cc.m, middle ciliated ring; 
young are hatched out with the for a es Racy) eet ee 
characters of the adult, but this 
is the case m all the Pulmonates— with the exception of 
the Siphonariidae which have a marine veliger larva—in the 
Opisthobranchs Cenia and Runcina, and in sundry Streptoneura 
such as Littorina and Lacuna among the Taenioglossa, and Purpura 
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 7vochus (Fig. 111), Patella, Fissurella, ete., 
and is the characteristic larval form in most opisthobranchiate 
3) 
Trane aay 
120. 
