82 
MOLLUSCA. 
of the branchiae previously to being l)rought to light*. All the Ace- 
phala are aquatic f . 
0 
ORDER I. 
ACEPHALA TESTACEA. 
Testaceous Acephala, or Acephala nith four branchial leafets 
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 lami- 
nae of the mantle ; forwards, and still betAveen these laminae are the 
four branchial leaflets, transversely and regularly striated by the ves- 
sels : the mouth is at one extremity, the anus at the other, and the 
heart towards the back ; the foot, when it exists, is inserted between 
the four branchiae. On the sides of the mouth are four triangular 
leaflets, Avhich are the extremities of the two lips, and serve as tenta- 
cula. The foot is a mere fleshy mass, the motions of Avhich ai’e 
effected by a mechanism analogous to that Avhich acts on the tongue 
of the Mammalia. Its muscles are attached to the bottom of the valves 
of the shell. Other muscles, Avhich sometimes form one mass and 
sometimes two, cross transversely from one valve to the other to keep 
them closed, but Avhen the animal relaxes these muscles, an elastic 
ligament placed behind the hinge opens the valves by its contraction. 
A considerable number of bivalves are provided with what is termed 
a hyssus, or a fasciculus of threads more or less loosely connected, 
which issues from the base of the foot, and by which the animal ad- 
heres 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 production is not thoroughly ascertained. Reaumur 
considered these threads as a secretion, spun and draAvn from the 
sulcus of the foot ; Poli thinks they are mere prolongations of tendi- 
nous fibres. 
* Some naturalists are of the opinion that the very minute bivalves, which in cer- 
tain seasons fill the external branchiae of the Anodunies and Mytilus, are not the progeny 
of those Mollusca, but a different and parasitic species. See, on this subject, the 
Dissertation of M. Jacobsen. The difficulty seems to be removed by the observations 
of Sir Ev. Home. 
f M. de Lamarck at first changed my name of Acephala into that of Acephalata. 
M. de Blainville forms a class, which he calls Acephalophora, from my Acephala 
and my Brachioj ola. 
+ M. de Lamarck, in his last work, has made his class of the Conchifera from 
my Testaceous Acephala; and M. de Blainville has converted the same into his order 
of the Acephalophora Lamellibranchiata : but it is always the same thing. 
