204 BKITISH MOLLUSKS. 



Class II. ACEPHALA — without head. 



The Acephala, or Headless mollusks, better known to conckolo- 

 gists as the Bivalves, abound in great variety in the sea, but in 

 fresh water they are very few in number. There are scarcely more 

 than half-a-dozen freshwater genera throughout the globe. There 

 are no land Acephala ; and none that respire air. All have shells. 

 The normal type of an Acephal is a compressed bag enclosing the 

 vital organs, including the branchiae, or gills, to which there are two 

 openings from behind, sometimes simple or j)outed, sometimes pro- 

 longed into siphons, separate or conjoined, one for inhaling the 

 surrounding fluid, the other for ejecting it; and there is an open- 

 ing in front for the passage of a burrowing, swimming, or adher- 

 ing organ, termed the foot. The Acephal having no head has no 

 tentacles or distinct eyes, but in several of the marine genera there 

 are pigments in certain parts of the body, or along the edge of the 

 mantle, which have very much the appearance of eyes, and are sup- 

 posed by some naturalists to be reaUy organs of vision. Internally, 

 on the side opposite to that of the siphonal orifices, there is an 

 opening to the stomach, edged with two or four lips, which has 

 been termed the mouth, but it is without jaws or tongue, or any kind 

 of muzzle or proboscis. Externally a pair of lobes, incorporated 

 with the integument, each secreting a shelly valve, constitutes the 

 mantle ; and the valves are connected dorsally by a cartilage, which 

 tends to open them in opposition to the action of internal adductor 

 muscles proceeding from the animal, which has a tendency to close 

 them. The hinge of the valves is, in most instances, strengthened 

 by interlocking protuberances on their dorsal margin, somewhat 

 ambiguously designated teeth. 



Our Dreissena is a very interesting example, high up in the series, 

 of an Acephalous mollusk in its normal condition of a closed in- 

 tegument, allied to the Ascidian. In a more advanced form, as in 

 the Anodon and TJnio, there is no closed integument, but a pair of 

 free mantle lobes, secreting, and corresponding with, the valves of 

 the shell, and wholly enveloping the body. 



