304 MOLLUSCS. 



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 {Cardiuvi) 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 ouc water from within the mantle-cavity ; many 

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

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

 is used for slow creeping movement. 



The food consists of Diatoms and other Algce, 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, starfishes, gas- 

 teropods, fishes, birds, and even mammals. 



Life-History. — We do not know very much about the development of 

 bivalves. In almost all the known cases, the eggs are fertilised in the 

 mantle-cavity by spermatozoa. drawn in with the water. The segmenta- 

 tion, influenced by the presence of some yolk, is unequal. A gastrula 

 is formed by invagination or overgrowth. In rare cases (Gateomma, 

 Kellia) the larvse issue as such from the genital apertures, but in most 

 they are sheltered for a while in the mantle-cavity, or gill-cavities (Uni- 

 onidse), or in special brood-chambers [Cyclas and Fisidium). The 

 larva developed from the gastrula is usually a trochosphere and thereafter 

 a veliger, and enjoys a. period of free-swimming before settling down. 

 In the fresh-water forms the velum is not developed, and the larva is 

 adapted by attaching threads, or by a grasping shell to resist being swept 

 away by currents. In the fresh-water mussels it becomes for a time 

 parasitic on the gills of a fish. 



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. 

 About 9000 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 

 two such muscles. _ Those which, from the shell-markings, seem to 

 have had an extension of the mantle into a protrusible tube or siphon, 

 were also of later origin. The present fresh-water forms are relatively 

 modern. Of all the fossil forms the most remarkable are large twisted 

 shells called Hippurites (Rudistte), whose remains are often very abund- 

 ant in deposits of the chalk period. 



Type of Lamellibranchiata or Bivalves : The Fresh- 

 water Mussel {Anodonta cygned). 



Mode of Life. — The fresh-water mussel lives in rivers and 

 ponds. It lies with its head end buried in the mud, or 



