90 CLASS ACEPHALA. 
THE FIRST ORDER OF ACEPHALA. 
TESTACEA. 
TESTACEOUS acephala, or acephala with four branchial 
leaflets, are, beyond all comparison, the most numerous: all 
the bivalves, and some genera of the multivalves, belong to 
this order. ‘Their body, which contains the liver and viscera, 
is placed between the two laminz of the mantle; forwards, 
and still between these laminz, are the four branchial leaflets, 
transversely and regularly striated by the vessels; the mouth 
is at one extremity, the anus at the other, and the heart to- 
wards the back; the foot, when it exists, is inserted between 
the four gills; on the sides of the mouth are four other trian- 
gular leaflets, which are the extremities of the two lips, and 
serve as tentacula ; the foot is amere fleshy mass, the motions 
of which are effected by a mechanism analogous to that of the 
tongue of the mammalia; its muscles are attached to the bot- 
tom of the valves of the shell: other muscles which form some- 
times one and sometimes two masses, cross transversely from 
one valve to the other, to keep them closed, but when the 
animal relaxes these muscles, an elastic ligament, placed be- 
hind the hinge, opens the valves by its contraction. 
A considerable number of bivalves are provided with what 
is termed a byssus, or a bundle of threads, more or less loosely 
connected, which issue from the base of the foot, and by 
which the animal adheres to various bodies. It uses its foot 
to direct the threads, and to agglutinate their extremities : it 
even reproduces them when cut; but the nature of the pro- 
duction is not thoroughly. ascertained. Reaumur considered 
