374 MOLLUSC A. 



[e.g., Teredo, Pholas, Lithodomus, Xylophaga), bore holes in stone or 

 wood, but we do not certainly know how ; in the great majority the foot 

 is used for slow creeping motion. 



The food consists of Diatoms and other Alg£e, Infusorians and other 

 Protozoa, minute Crustaceans and organic particles, which the ciliary 

 action of the gills carries from the posterior end of the shell to the 

 mouth. The bivalves are themselves eaten by worms, star fishes, gas- 

 teropods, 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 sper- 

 matozoa drawn in with the inhalent water, and are subsequently sheltered 

 within the body during part of the development. In the Unionidae the 

 embi^os are retained within the cavities of the outer gills ; in Cyclas 

 and PisidimiL 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 invagina- 

 tion or by overgrowth, the two cases being connected 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 Dreissenapoly- 

 morpha in which the habit is recently acquired, do not possess free 

 swimming larvce ; this must be regarded as an adaptation. 



Past History of Bivalves. — Even in Cambrian rocks, which w"e may 

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

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

 About gooo extinct and 5000 living species were catalogued some years 

 ago, so that we evidently have our full share now\ Those with one 

 closing muscle to the shell seem to have appeared after those which have 

 tW'O such muscles. Those W'hich, from the shell markings, seenr to have 

 had an extension of the mantle into a protrusible 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 

 Hippnrites (Rudistre), 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 swimmitig 

 Molluscs. Part of the foot has come to surround the head, 

 and is divided into numerous ^' arms" bearing tentacles or 

 S2(ckers. Another part forms a par-tial or complete tube — the 

 ^'siphon" or "fimnel" — through which water is forcibly 

 expelled from the mantle cavity, driving the animal backwards. 

 The muscular mantle flap which shelters the gills is posterior 

 in position ; the visceral hump shows no trace of spiral coiling, 

 but is elongated in a direction anatomically dorsal and posterior. 



