ACEPHALA TESTACEA. 259 



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 dis- 

 tributes 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. 



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 body, which contains 

 the liver and viscera, is placed between the two 

 laminaD of the mantle; forwards, and still between 

 these laminae are the four branchial leaflets, transversely and regularly striated 

 by the vessels: the mouth is at one extremity, and the heart towards the 

 back; the foot, when it exists, is inserted between the four branchiae. On 

 the sides of the mouth are four triangular 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 ligament placed 

 behind the hinge opens the valves by its contraction. 



A considerable number of bivalves are provided with what is termed a 

 byssiis, or a bundle of threads more or less slender, which issues from the base 

 of the foot, and by which the animal adheres to various bodies. It uses its 

 foot to direct the threads and to agglutinate their extremities ; it even 

 reproduces them when cut, but the nature of the production is not thoroughly 



s 2 



