GASTEROPODA CYCLOBRANCHIATA. 
81 
coriaceous, and furnished either with a naked skin or little scales, 
which give it the appearance of shagreen, or with spines, hairs, or 
setaceous fasciculi. Under these edges, on each side, is a range of 
lamellar, pyramidal branchiae ; and before, a membranous veil on 
the mouth supplies the want of tentacula. The anus is under the 
posterior extremity. The heart is situated behind, on the rectum, 
the stomach is membranous, and the intestine very long and greatly 
contorted. The ovary is situated over the other viscera, and appears 
to open on the sides by two oviducts. 
A few small species are found on the coast of F ranee ; very 
large ones abound in the seas of hot climates *. 
CLASS IV. 
ACEPHALA. 
The Acephala have no apparent head; hut a mere mouth concealed 
in the bottom, or between the folds of their mantle. The latter is 
almost always doubled in two, and encloses the body as a book is 
clasped by its cover ; but it frequently happens, that, in consequence 
of the two lobes uniting before, it forms a tube; sometimes it is closed 
at one end, and then it represents a sac. This mantle is generally 
provided Avith a calcareous bivalve, and sometimes multivalve shell, 
and in two genera only is it reduced to a cartilaginous, or even mem- 
branous nature. The brain is over the mouth, where Ave also find one 
or tAVO other ganglia. The branchiae usually consist of large lamellae 
covered Avith vascular meshes, under or betAA'een Avhich passes the 
Avater ; they are more simple, hoAvever, in the genera Avithout a shell. 
From these branchiae the blood proceeds to a heart, generally unique, 
Avhich distributes it throughout the system, returning to the pulmo- 
nary artery Avithout the aid of another ventricle. 
The mouth is ahvays edentated, and can only receive the molecules 
brought to it by the Avater : it leads to a fii'st stomach, to Avhich there 
is sometimes added a second ; the length of the intestines is extremely 
various. The bile is throAvn by several pores into the stomach, Avhich 
is surrounded by the mass of the liver. 
All these animals fecundate themselves, and in several species, the 
young ones, Avhich are innumerable, pass some time in the thickness 
* The Chitonelli of Lamarck, and all the species of Chiton of authors, 
should be left in this genus, of which M. de Blainville has thought proper to make 
a separate class, called Pola'plaxiphora, supposing that it leads to the Articulated 
Animals. 
VOL. III. G 
