GASTEROPODA CYCLOBRANCUIATA. 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 France ; very 

 large ones abound in the seas of hot climates *. 



CLASS IV. 



ACEPHALA. 



The Acephala 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 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 with 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 we also find one 

 or two other ganglia. The branchiae usually consist of large lamellae 

 covered with vascular meshes, under or between which passes the 

 water ; they are more simple, however, in the genera without a shell. 

 From these branchiae the blood proceeds to a heart, generally unique, 

 which distributes it throughout the system, returning to the pulmo- 

 nary art. ry without the aid of another ventricle. 



The mouth is always edentated, and can only receive the molecules 

 brought to it by the water: it leads to a first stomach, to which there 

 is sometimes added a second ; the length of the intestines is extremely 

 various. The bile is thrown by several pores into the stomach, which 

 is MUTiMuulrd by the mass of the liver. 



All these animals fecundate themselves, and in several species, the 

 young ones, which are innumerable, pass some time in the thickness 



l!u ( UMONKLLI of Lamarck, and all tin- spi-ries of CHITON of authors, 

 shonM l>r left in this eenus, of which M. de Hlainvillc has thought proper to make 

 a M-parate class, called POLYPLAXIPHORA, supposing that it leads to the Articulated 

 Animals. 



VOL. HI O 



