80 SECTIONAL ADDRESSES. 



Nemertine may have arisen. However, without ranging further afield, 

 these few examples may perhaps sufiice to illiistrate several important 

 propositions ; 



(1) the larva has a double task to perform, viz. to distribute the species 

 and to grow up into the adult ; 



(2) of these tasks the first is essential, and the second subsidiary — to 

 be undertaken only so far as the larval resources permit ; 



(3) the performance of the two tasks together requires the maintenance 

 of an equilibrium between the locomotive efficiency of the larva and the 

 adult weight to be carried ; 



(4) the locomotive adaptation of the larva may proceed on new 

 lines, paying no respect to phylogeny, and culminating in some kind of 

 metamorphosis ; 



(5) the modification of the larva in this way need not affect the 

 organisation of the adult, since the casting of the most hypertrophied of 

 ciliated girdles involves only slight processes of subsequent repair. 



When we pass from the more primitive and ancient groups of Mollusca 

 to the more modern ones, the larva no longer hatches as a simple 

 trochosphere, but is provided with a shell and foot from the first, and the 

 simple girdle of cilia which constituted the prototroch is replaced by a 

 much more powerful organ, the velum. This applies to all except the 

 lowest members of the Azygobranch Gastropodaand to all Filibranch 



Fig. 3. — Larvse of Bivalves. 

 A, Yoldia {Protobranch) ; B, Ostrea, and C, Dreissensia (Eulamellibraneha), 



and Eulamellibranch Bivalves. The velum is only a special development 

 of the prototroch, but by being stretched out at the edge of an extended 

 disk or bi- or tri-lobed frill, the locomotive cilia of the girdle are greatly 

 multiplied in number and power. The larva is the familiar Veliger, though 

 it would be well to restrict this term to the Gastropod larva, and to 

 distinguish the Bivalved form of it by a separate name, e.g. Rotiger, from 

 the wheel-like form of its ciliated disk. Both the ciliated arms of the 

 Veliger and the disk of the Rotiger can be protruded freely from the 

 shell and as easily and completely withdrawn inside it. 



There are of course many Gastropods and Bivalves in which, even 

 under marine conditions, the free-swimming larval stage has been 

 secondarily reduced in association with a marsupial or incubatory mode 

 of development. Under these conditions the velum or ciliated disk never 



