OCR Text |
Show 390 MOLLUSCA. CLASS IV. ACEPHALA. The Acephala have no apparent head ; but a mere mouth concealed 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 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 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 branchire usually consist of large lamellre covered with vascular meshes, under or be· tween which passes the water; they are more simple, however, in the genera without a shell. From these branchire the blood J>roceeds to a heart, generally unique, which distributes 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 stomach, to which there is sometimes added a second; the length of the intestines is extremely various. The bile is thrown by several JlOres into the stomach, which is surrounded by the mass of the liver. All these animals fecundate themselves, and in several species, the young ones, which are innumerable, pass some time ACEPHALA. 391 in the thickness of the branchire previously to being brought to light(l ). All the Acephala are aquatic(2). ORDER I. ACEPHALA TESTACEA. Testaceous Acephala, or Acephala with four branchialleaflets( 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 laminre of the mantle; forwards, and still between these laminre 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 towards the back ; the foot, when it exists, is inserted between the four branchire. 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 {1) Some naturalists are of the opinion that the very minute bivalves, which in certain seasons fill the external branchia: of the .!l.nodonte3 and Mytilus, are not the progeny of those Mollusca., but a different and parasitic species. See, on this subject, the Dissertation of M. Jacobsen. The difficulty seems to be removed by the observations of Sir Ev. Home. (2) M. de Lamarck at first changed my name of .!l.cepltala into that of .!l.cep!talata. M. de Blainville forms a class, which he calls AcEPH.ALO.PUORA, from my .!l.cepltala and my Bracldopoda. (3) M. de Lamarck, in his last work, has made his class of the CoNCHIFER"- from my Tutaceous .!l.cephala; and M. de Blain ville has converted the same into his order of the AcEPHA.LoraonA LAM:&LLIDRANcau.·.u: but it is always the same thing. |