CLASS ACEPHALA. 89 
FOURTH CLASS OF THE MOLLUSCA. 
ACEPHALA. 
THEY have no apparent head, but a mere mouth concealed in 
the bottom, or between the folds of their mantle: the latter is 
almost always doubled, and encloses the body, as a book is 
clasped by its cover ; but it frequently happens that in conse- 
quence of the two lobes uniting before, it forms atube. Some- 
times it is closed at one side, and then it represents a sac. 
This mantle is generally provided with a calcareous bivalve, 
and sometimes multivalve shell, and in two genera only is it 
reduced to a cartilaginous or even membranous nature. The 
brain is over the mouth, and there are also one or two other 
ganglia; the gills usually consist of large leaves, covered with 
vascular net-work, under or between which the water passes ; 
they are more simple, however, in the genera without a shell. 
From these gills the blood proceeds to the heart, generally 
simple, which distributes it throughout the system, returning 
to the pulmonary artery, without the aid of another ventricle. 
The mouth is always toothless, and can only receive the 
molecules brought to it by the water; it leads toa first stomach, 
to which is sometimes added a second; the length of the in- 
testines is very various; the bile is thrown into the stomach 
by several pores, and the stomach itself is surrounded by the 
mass of the liver. 
__ All these animals fecundate themselves, and in several tes- 
tacea the young ones, which are innumerable, pass some time 
in the thickness of the gills, previously to being brought to 
light. All the acephala are aquatic. 
