370 



MOLLUSCA. 



that, in consequence of the two lohes uniting in front, the cloak 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 multivalve, shell ; and in two families 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 branchiae usually consist of large lamellae, 

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

 shelled species the young, which are innumerable, are retained for some time between 

 the laminae of the [external] branchiae before they are expelled.* All the Acephales are 

 aquatic. 



THE FIRST ORDER OF THE ACEPHALES. 



THE TESTACEOUS ACEPHALES f (or a. with four branchial leaflets). 



They are beyond comparison the most numerous. 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 four 

 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 laminae at the sides of the 

 mouth, which are the extremities of two lips, and are used as tentacula. 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, which form 

 sometimes one, sometimes two masses, go straight across from one valve to the 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 byssus, that is, a bundle of more 

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

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

 and to glue them there : and it can reproduce them when they have been cut away ; but 

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

 from a secretion, and moulded in the groove of the foot. Poli thinks them to be merely pro- 

 longations of tendinous fibres. 



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

 times composed of a greater or less number of teeth and laminae, 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 [subspiral] 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 maintained that the minute 

 bivalves which, in certain seasons, load the external branchiae of the 

 freshwater Mussel, are not the foetal young, but parasites of diffe- 



rent species. This opinion is now generally considered as erro- 

 neous. 



t The class Conchtfera of M. de Lamarck. 



