GENERAL NOTES ON LAMELLIBRANCHS. 427, 
slowly up from estuary to river, from river to lake; Dvrezssensia 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 Cyc/as ; (4) on the 
other hand, it is more probable that the fresh-water mussels (U/7zo, 
Anodonta, etc.) are relics of a fauna which inhabited former inland 
seas, of which some lakes are the freshened residues. 
Between the active Zzma 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 (Cardizum) uses its bent foot to take small jumps on the 
sand ; the razor-fish (Soe) not only bores in the sand, but may swim 
backwards by squirting out water from within the mantle cavity ; many 
(e.g. Leredo, 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 Algze, Infusorians and other 
Protozoa, minute Crustaceans and organic particles, which the cilia 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 (Montacutidz) are known,—Montacuta 
on heart-urchins, Z7zova/va in the gullet of Synaptids, Sczoderetza on a 
sea-urchin, and Jousseaumdella on a Sipunculid. 
Life history.—The eggs are sometimes laid in the water, either 
freely or in attached capsules, or 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 Unionidz the 
embryos are retained within the cavities of the outer gills; in Cyclas 
and Pzszdium there are special brood-chambers at the base of the gills. 
In Cyc/as the embryos are nourished by the maternal epithelial cells. 
Seginentation 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 Dredssensia polymorpha, in which the habit is recently acquired, do 
not possess free-swimming larvze ; this must be regarded as an adapta- 
tion. 
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 
remarkable are large twisted shells, called Azppurdtes (Rudistze), whose 
remains are often very abundant in deposits of the chalk period. 
