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 ** and -f- inner and outer s 111 - 

 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; tn., mantle; 



