GASTEROPODA CYCLOBRANCHIATA. 271 



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 con- 

 cealed 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 con- 

 sequence 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 multi- 

 valve, shell, and in two genera only is it reduced to a cartilaginous, or 

 even membranous 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 branchi the blood proceeds 

 to a heart, generally unique, which distributes it throughout the sys- 

 tem, returning to the pulmonary artery without the aid of another 

 ventricle. 



The mouth is always edentated, and can only receive the mole- 

 cules brought to it by the water. 



All the Acephala are aquatic. 



ORDER I. 

 ACEPHALA TESTACEA. 



Testaceous Acephala, or Acephala with four branchial leaflets, 

 are beyond all comparison the most numerous. All the bivalves, 

 and some genera of the multivalves belong to this order. Their 



