ON ACEPHALA. 401 
composed of three layers of fibres, directed according to 
length, breadth, and depth, imparts to the animal the faculty 
of changing form in these three dimensions. The anodontes 
are viviparous. ‘Thousands of young ones are frequently 
found, in winter only, between the plates of each gill. They 
may be seen distinctly with a good microscope, opening and 
closing their shell. All the anodontes are fluviatile. When 
they want to walk, they place their shell level, put out their 
foot, and crawl something like snails. It has been said that 
they swim by striking the water with their valves. They are 
most commonly found sunk in mud, the aperture of the valves, 
and the obtuse extremity which corresponds to the mouth, 
directed upwards. They may serve as food, but are in no 
kind of estimation. 
There is nothing to induce us to mention the UNIO here, 
except the circumstance of some of the species producing 
pearls, on which we have already sufficiently enlarged. The 
species are very numerous. 
The mollusca of the genus CHAMA live attached to rocks, 
like the oysters, by one of their valves, also to corals, and 
even to other shell-animals. This often injures their develop- 
ment, and renders it more or less irregular. They are fre- 
quently found in numerous groups, pressed one upon the 
other. They are thus cemented so strongly, that they cannot 
_ be detached without breaking the shell. The adherent valve 
assumes, like that of the anouriz, and oysters, to a certain 
point the form of the bodies on which it reposes. It is 
usually rather irregular, and much less coloured than the 
other. From this mode of existence, we may easily conceive 
that the chamz make but little use of their foot. 
Almost all the CARDIA, to which we now pass, live in the 
sea; a very small number of species inhabit the fresh-water. 
Some of them remain remote from the coasts: but the majority 
prefer sandy shores, where they remain concealed in the sand 
von. XI. pd 
