-CONCHIFERA The molluscous animals which are destitute of a head, were separated at an early period, by Cuvier, into a distinct order, to which he gave the name of Acephala. Subsequent observations having pointed out the charac ter of being acephalous, as common to animals which differ widely from one another in the arrangement and disposition of their other organs, it became necessary to introduce into the system a more definite method of ar rangement. Accordingly, Lamarck, in his a Histoire .,Vattirelle des Animaux sans Vertebres," v. p. 411, insti tuted the class CONCHIFERA, which includes nearly all the inhabitants of the bivalve testacea, or those whose gills are in the form of leaves, four in number, and dis posed in pairs externally, on each side the abdomen, and within the cloak. As the bivalve shells were long known to naturalists under the denomination Concha, the pro priety of the term to designate the class, employed by Lamarck, is sufficiently obvious.
The common integuments of the Conchifera, consist of the cloak and shell. The cloak forms two leaves, one on each side the body of the animal, united behind. This cloak is, in some families, open in front, while in others, it is united ; perforated, however, by holes, or tu bular elongations, termed syphons, for admitting water and food. Corresponding with the two sides of the cloak, are the two valves of the shell. These valves are like wise united behind with an elastic ligament, which aids the animal in opening and shutting them. The shells are attached to the animal by the anterior margin of the cloak, which adheres to the margin of the shell, and by the adductor muscle. This muscle, which passes across the body from one valve to another, brings the valves, by its contractions, into contact, at their free edges, at the same time that the ligament is compressed or stretched, according as it is internal or external. When the mus cle is relaxed, the ligament exerts its power, and opens the valves ; and along with the valves, the cloak to which they are attached. The adductor muscle is in some fa milies divided, and the two portions separated from each other.
Locomotion is performed in some to a limited extent, by suddenly opening and shutting the valves. Ili ge neral, however, those species which shift their place are furnished with a muscular projection from the body, capable of changing its shape, and attaching itself to foreign bodies, termed a foot. The base of the foot is usually attached by two or more tendinous filaments to the shell. The byssus issues from a muscular body, like wise connected by filaments with the shell, and is fixed to other bodies. The foot is supposed to be the organ which spins this thread ; but its mode of formation is involved in obscurity. While some are permanently fixed, and others are capable of moving from one place to another, there are a few which prefer a residence in different substances in which they have excavated a habitation. These last are termed Borers. It was sup posed by many that the animal secreted a liquor, with which it dissolved the bodies into which it penetrated ; but the sagacious Reaumur soon ascertained that the boring was performed by means of a rotatory movement of the larger valves. M. Fleurieu Bellevue states, that the calcareous stone, in which the Rupellaria lithophaga is found, is often discoloured in the immediate neigh bourhood of its recess. This may arise from the secre tions of the animal, or even from the stagnant sea water in the hole, and not from the action of the phosphoric acid, or any other solvent supposed to be employed by the animal. These solvents would act equally on the shell as on the calcareous rock. But the borers are not confined to calcareous rocks : they also lodge in slate clay, and other argillaceous strata. This is very often the case with the Pholades. But this character can never be extensively employed in the distribution of genera, as the same species which, at one time, may he found im bedded in stone, will be observed, at another, seated among the roots of seaweed, or buried among gravel.