ACEPHALA. 391 



in the thickness of the branchiae previously to being brought 

 to light(l). All the Acephala are aquatic(2). 



ORDER I. 



ACEPHALA TESTACEA. 



Testaceous Acephala, or Acephala with four branchial leaf - 

 lef.s{3), are beyond all comparison the most numerous. All 

 the bivalves, and some genera of the multivalves belong to 

 this order. Their body, which contains the liver and viscera, 

 is placed between the two lamintB of the mantle; forwards, and 

 still between these laminse are the four branchial leaflets, 

 transversely and regularly striated by the vessels: the mouth 

 is at one extremity, the anus at the other, and the heart to- 

 wards the back ; the foot, when it exists, is inserted between 

 the four branchiae. On the sides of the mouth are four tri- 

 angular leaflets, which are the extremities of the two lips, 

 and serve as tentacula. The foot is a mere fleshy mass, the 

 motions of which are effected by a mechanism analogous to 

 that which acts on the tongue of the Mammalia. Its muscles 

 are attached to the bottom of the valves of the shell. Other 

 muscles, which sometimes form one mass and sometimes two, 

 cross transversely from one valve to the other to keep them 

 closed, but when the animal relaxes these muscles, an elastic 



(1) Some naturalists are of the opinion that the very minute bivalves, which in 

 certain seasons fill the external branchia: of the Jlnodontes and Mt/tilus, are not the 

 progeny of those iNIollusca, but a different and pai-asitic species. Sec, on this 

 subject, the Dissertation of M. Jacobsen. The difficulty seems to be removed by 

 the observations of Sir Ev. Home. 



(2) M. de I-amarck at first changed my name oi Acephala into that of Acephalata. 

 M. de Blainville forms a class, which he calls AcEPiiALopiionA, from my Acephala 

 and my Brachiopoda. 



(3) M. de Lamarck, in his last work, has made his class of the CoNciiiFEnA from 

 my Testaceous Acephala,- and M. de Blainville has converted the same into his order 

 of the AcEPHAi-oPHonA LAMELtiBRAXcHfATA: but it is always the same thing. 



