MOLLUSCS 



405 



for they are now unsegmented, and very unlike annelids in most 

 respects. It is impossible to discuss so difficult and technical a 

 problem here, but we should do well to look with suspicion upon 

 the theory in so far as it is based upon the occurrence of a trocho- 

 sphere larva in both annelids and molluscs. The peculiarities of 

 this kind of larva have arisen as the result of adaptation to a free- 

 swimming life, and they are of so simple a kind that they may 

 quite conceivably have been evolved independently in half a dozen 

 different distinct groups of animals, which have had the same life- 

 problems to solve. 



The Mail- Shell larva gradually undergoes a metamorphosis, 

 ultimately losing its ciliated bands, develop- 

 ing shell -plates, foot, &c., and giving up 

 swimming for creeping. 



Bivalve Molluscs (Lamellibranchia). 

 — In most of the marine bivalves which have 

 so far been studied the eggs are simply 

 ejected into the surrounding water, and 

 there pass through the stages of their de- 

 velopment. This is the case, for example, 

 in the American Oyster {Ostrea Virginiana) 

 and the Edible Mussel {Mytilus edulis). But 

 in some marine and most freshwater forms 

 the eggs have a better chance of escaping 

 destruction, for they hatch out between the 

 protective shells of the parent. For such an arrangement the 

 structure of a bivalve offers special advantages, as will be 

 gathered from the diagrammatic cross -section represented in 

 fig. 926. Hanging down on either side, and covering the body 

 like the flap of a coat, is a mantle-lobe, the outer side of which 

 bears a shell. The space between body and mantle -lobes is 

 known as the mantle-cavity, and into this two gill-plates project 

 on either side. Each plate consists of two layers, between 

 which is a space divided into a series of tubes by vertical par- 

 titions. As a matter of fact, the gills of bivalves vary greatly in 

 different species, and the statement just made applies to forms 

 in which they are large and complex. Here, then, are a number 

 of sheltered spaces within which eggs can be incubated, and a 

 further advantage is secured by the fact that water is constantly 

 streaming through the mantle-cavity and gills, in relation to the 



Fig. 926. — Diagrammatic Cross- 

 Section through a Bivalve Mollusc. 

 /., Ligament; s., shell; w., mantle; 

 i.g. and o.g., inner and outer gill- 

 plates. 



