380 MOLLUSC A. 



their range extends from the sand of the shore to great depths. They 

 occur in all parts of the world, though only a few forms, like the edible 

 mussel (Mytilus edulis), can be called cosmopolitan. Some, such as 

 oysters, can be accustomed to brackish water. The fresh-water forms 

 may have found that habitat in two ways — (a) a few may have crept 

 slowly up from estuary to river, from river to lake ; Dreissenia poly- 

 morpha has been carried on the bottom of ships from the Black Sea to 

 the rivers and canals of Northern Europe ; and it is likely that aquatic 

 birds have assisted in distributing little bivalves like Cyclas ; (b) on the 

 other hand, it is more probable that the fresh-water mussels (Unio, 

 Anodonta, etc.) are relics of a fauna which inhabited former inland 

 seas, of which some lakes are the freshened residues. 



Between the active Lima and Pecten, which swim by moving their 

 shell valves and mantle flaps, and the entirely quiescent oyster, which 

 has virtually no foot, there are many degrees of passivity, but most 

 incline towards the oyster's habit. Of course, there is much internal 

 activity, especially of ciliated cells, even in the most obviously sluggish. 

 The cockle (Cardium) uses its bent foot to take small jumps on the 

 sand ; the razor-fish (Solen) not only bores in the sand, but may swim 

 backwards by squirting out water from within the mantle cavity ; many 

 (e.g. Teredo, Pholas, Lithodomus, Xylophagd) bore holes in stone or 

 wood ; in the great majority the foot is used for slow creeping motion. 



The food consists of Diatoms and other Alga;, Infusorians and other 

 Protozoa, minute Crustaceans and organic particles, which the cilia of 

 the gills sweep from the posterior end of the shell to the mouth. The 

 bivalves are themselves eaten by worms, starfishes, gasteropods, fishes, 

 birds, and even mammals. 



Life history. — The eggs are sometimes laid in the water, either 

 freely or in attached capsules, or, more frequently, they are fertilised by 

 spermatozoa drawn in with the inhaled water, and are subsequently 

 sheltered within the body during pait of the development. In the 

 Unionidse the embryos are retained within the cavities of the outer 

 gills ; in Cyclas and Pisidium there are special brood-chambers at the 

 base of the gills. In Cyclas the embryos are nourished by the maternal 

 epithelial cells. Segmentation is always unequal ; a gastrula may be 

 formed by invagination or by overgrowth, the two cases being con- 

 nected by a series of gradations. A trochosphere stage is more or less 

 clearly indicated, being most obvious in cases where the eggs are laid in 

 the water. The free-swimming trochosphere becomes a veliger, and 

 this is modified into the adult. The fresh-water forms, with the 

 exception of Dreissenia polymorpha, in which the habit is recently- 

 acquired, do not possess free-swimming larvte ; this must be regarded as 

 an adaptation. 



Past history of bivalves. — Even in Cambrian rocks, which we 

 may call the second oldest, a few bivalves have been discovered ; in the 

 Upper Silurian they become abundant, and never fall off in numbers. 

 Those with one closing muscle to the shell seem to have appeared after 

 those which have two such muscles. Those which, from the shell 

 markings, seem to have had an extension of the mantle into a pro- 

 trusible tube or siphon, were also of later origin. The present fresh- 

 water forms were late of appearing. Of all the fossil forms the most 



