LIFE-HISTORY OF BIVALVES 48 1 



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 ; Dreissensia 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 {Unto, 

 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 cihated 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, 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 Algae, Infusorians and other 

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

 the gills and palps sweep towards the mouth. The bivalves are them- 

 selves eaten by worms, starfishes, gasteropods, fishes, birds, and even 

 mammals. 



Several commensal bivalves (Montacutidae) are known — Montacuta 

 on heart -urchins, Entovalva in the gullet of Synaptids, Scioberetia on a 

 sea-urchin, and J ousseaumiella on a Sipunculid. 



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

 freely or in attached capsules, or they are fertihsed 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 Pisidimn there are special brood-chambers at the base of the gills. 

 In Cyclas the embr^^os are nourished by the maternal epithehal 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 Dreis- 

 sensia polymorpha, in which the habit is recently acquired, do not 

 possess free-swimming larva^ ; 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 



31 



