390 MOLLUSC A. 



CLASS IV. 



ACEPHALA. 



The Acephala have no apparent head ; but a mere mouth 

 concealed in the 1)Ottom, 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 hap- 

 pens, 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 be- 

 tween which passes the water; they are more simple, howe- 

 ver, in the genera without a shell. From these branchiai the 

 blood proceeds to a heart, generally unique, which distrib- 

 utes it throughout the system, returning to the pulmonary 

 artery 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 sto- 

 mach, 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 surrounded by the 

 mass of the liver. 



All these animals fecundate themselves, and in several spe- 

 cies, the young ones, which are innumerable, pass some time 



