408 PHYLUM MOLLUSC A. 



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 (Cardittm] uses its bent foot to take small jumps on the 

 sand ; the razor-fish (Soleti) 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, Xylophaga) 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 Algre, 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 part of the development. In the 

 Unionidae 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 imagination 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 larvae ; 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 oft 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 

 remarkable are large twisted shells, called Hippitrites (Rudistae), whose 

 remains are often very abundant in deposits of the chalk period. 



Class V. CEPHALOPODA. Cuttlefish. 

 Examples. Sepia, Octopus, Loligo, Nautilus. 



The Cephalopods are bilaterally symmetrical free-swimming 

 Molluscs. The head is surrounded by numerous " arms " 



