370 MOLLUSCA. 



that, in consequence of the two lobes uniting in front, the cloal< forms a tube, or a sac 

 when it is only closed at one end. This cloak is generally provided with a calcareous 

 bivalve, and sometimes multivalvc, shell ; and in two families only is it reduced to a 

 cartilaginous, or even membranous nature. I'he brain is over the mouth, where we 

 also find one or two other ganglia. The branchife usually consist of large lamella;, 

 covered with vascular network, under or between which the water passes : they are 

 more simple, however, in the genera without a shell. From these branchiae the blood 

 proceeds to a heart, generally single, wdiich distributes it throughout the system, 

 returning to the pulmonary artery without the aid of another ventricle. 



The mouth is always toothless, and can only seize upon such particles as the water 

 floats within reach. It leads into a first, and sometimes a second, stomach : the intes- 

 tine varies much in length. The bile is poured, generally by several pores, into the 

 stomach, wdiich the liver surrounds. All fecundate themselves ; and in several of the 

 shelled species the young, which are innumerable, are retained for some time betw'een 

 the lamina; of the [external] branchire before they are expelled.* All the Acephales are 

 aquatic. 



THE FIRST ORDER OF THE ACEPHALE.S. 



THE TESTACEOUS ACEPHALESf (or A. with fodr branchial leaflets). 



They are beyond comparison the most niunevous. All bivalve shells, and some kinds of 

 multivalves, belong to them. Their body, which includes the liver and the viscera, is placed 

 between the two layers of the cloak; and in front, still between the same layers, are the fom- 

 branchial leaflets, regularly striated crosswise by the vessels. The mouth is at one extremity, 

 the anus at the other. The heart is towards the back. The foot, when there is one, is 

 attached between the four branchiae. There are four triangular laminfe at the sides of the 

 mouth, which are the extremities of two lips, and are used as tcntacula. The foot is merely 

 a fleshy mass, moved by a mechanism similar to that of the tongue of mammiferous animals : it 

 has its muscles fixed in the bottom of the valves of the shell. Other muscles, whieli form 

 sometimes one, sometimes two masses, go straight across from one valve to tlie other, to keep 

 them closed ; but when the animal relaxes these muscles, an elastic ligament situated behind 

 the hinge opens the valve by its contraction. 



A considerable number of Bivalves possess what is called a bi/ssiis, that is, a bundle of more 

 or less delicate filaments issuing from the base of the foot, and by means of which tlie animal 

 fixes itself to foreign bodies. It employs the foot to guide the filaments to the proper |ilacc, 

 and to glue them there : and it can reproduce them wdien they ha\'e been cut aw a^' ; but 

 nevertheless their true nature is not yet well ascertained. Reaumur believed them to be spun 

 from a secretion, and moulded in the groove of the foot. Poll thinks them t(j lie merely pro- 

 longations of tendinous fibres. 



The shell consists of two valves connected bj' a hinge, which is sometimes simple, and some- 

 times composed of a greater or less number of teeth and lamiuEe, that are received into cor- 

 responding sockets and cavities. In a few genera, some supernumerary pieces are laid over 

 the hinge. In general the valves have, leaning over the hinge, a prominent [siibspiral] part, 

 which is named the summit, or the nates. 



In the greater number the valves close perfectly when the animal chooses to draw them 



* Some naturalists, as Jacobson, have maintaiDed that the minute [ rent Kpecies. This opinion is now generally consiJered as eiro. 

 biralvcs which, in certain seasons, loaJ the external branchite of the neons, 

 freshwater Mussel, are not the f.Ktal young, but [jarasites of dilTe- I f Tlie class Cun.ltiJeTn of I^I. de Lamarck. 



