In the Monomyaria the nervous system is in general less perfectly developed than in the Dimyaria. It is not quite symmetrical, and the posterior are not larger than the anterior ganglions. The nervous cords, too, are much more slender, and not nearly so easy of de monstration as in the Dimyaria; it was not without difficulty that we ,discovered them in the common Oyster, the Pecten and the Spon dylus. Poli has said nothing upon the nervous system of these genera. Our own researches in quest of it were perfectly fruitless at first ; but having bethought us that in the Dimyaria the nervous cords of the labial palps were always to be discovered without difficulty, we sought for the same filaments in the Mono inyaria, and were lucky enough to find them; these led us by-and-bye to the anterior gan glions, and by degrees to the detection of the entire nervous system. The anterior ganglions in the Monomyaria are extremely small ; they send a principal filament to each of the palps; a cord proceeds from them to the anterior part of the mantle which covers the mouth ; another' runs from the ganglion of one side to that of the other, passing above the cesophagus; and from the posterior angle several branches are detached to the liver, the stomach, and the branchiw. Among these there iS one, and sometimes two, which, resting on the internal aspect of the central muscle, bend obliquely over its surface, and finaliy unite occasionally to form a small posterior ganglion. This gan glion sends branches to the heart, to the ovary, and to the posterior parts of the mantle. The parallel cords traverse the thin part of the man tle, sometimes radiating in a slight degree, and divide into numerous branches within its thick margin and the tentacular ciliary processes that fringe it. There is one among the monomyary genera, the nervous system of which we have not been able to study with due attention; this is the genus Lima. From what we have seen of it, however, it would appear that the ner vous system in this genus is every way as perfectly symmetrical as in the Dimyaria. But before admitting this as a fact definitively, it were necessary to have verified its accuracy at least several times, which we have as yet had no opportunity of doing.
When we consider the great simplicity of the nervous system of the acephalous mol lusca, we can only conceive these animals endowed with sensibilities extremely obscure, and with instincts extremely limited. No especial organ of sense can be detected among them, unless perhaps it be that of touch, which appears to reside in every part of the body and of the mantle and probably also the sense of taste, of which in all likelihood the maxillary palps are the organ. The manlier of existence of these animals is in perfect accordance with the great simplicity of their nervous system. Many genera live attached to submarine ob jects, either by the shell immediately or by means of a byssus, taking no pains to avoid or to protect themselves from danger, and giving no sign of existence but by opening and shut ting their shells : they shut them when any foreign body comes in contact with their mantle ; and they open them to admit the water which brings suspended in it the nutri tious particles which they seize upon for their subsistence, and which is in itself necessary for the purposes of -respiration. Among the acephalous mollusca which are not fixed in the manner of those now mentioned, those which have no siphon, or which have this part very short, live at the bottom of the sea, in spots covered with sand or mud, amidst which they burrow by means of the foot, and support themselves in an oblique position by resting upon the half-open valves of their shell. The
acephalous mollusca again, which are furnished with a siphon, almost all bury themselves more or less deeply ainid the sand or the mud of the bottom, contenting themselves with an ascend ing or a descending motion, the latter sufficient in the moment of danc.,,er to gain the limits of their retreat, the former to enable them to protrude the free extremity of their siphon when they would establish the current of water necessary to their nutrition and respi ration. It is easy to imagine that among ani mals whose functions of external relation are so limited, the nervous system must continue extremely sirnple, a fact which could in some measure be predicated from observation of the habits of the extensive class whose structure and economy we are now engaged in consi dering-.
Qj the skin and its appendages.— The mantle.—The acephalous mollusca are enve loped by two very thin fleshy laminw, which are seen covering or closely applied to the whole of the inner surface of the shell ; this is the part to which the name of mantle has been given (e, fig. 359 ; a, a, fig. 360). This name has been very appropriately given to this cuta neous envelope, for it appears to be applied over the back of the animal, and to be extended over the lateral parts, to rneet by its edges along the anterior middle aspect of the body. The mantle is composed of two parts generally equal, or nearly equal, each of which has been designated one of' its lobes. In the natural position of the animal, one of these lobes is in relation with its right side, the other in relation with its left side; they adhere intimately to the superior and posterior part of the body; they become free at the origin of the branchim, and form around the whole inferior part of the animal a cavity of various dimensions, within which the abdominal mass, the foot, and the branchite are included. It is in this palleal sac that the animal establishes a current of water, destined to minister to the function of respiration, and to carry towards the mouth the alimentary particles with which it is fed. The median parts of the lobes of the mantle are ex tremely thin and transparent, and a great number of vessels (c, fig. 362), and a few nervous filaments (7, 8,fg.360) are perceived ramifying through their substance, and running towards the anterior and inferior edges. These edges, which extend as far as those of the shell, are thickened, and it is at the point where the thickening begins that the mantle adheres to the shell by means of a great number of minute muscles (1,1, fig. 347; d, fig. 362), which leave a linear impression upon it. The thickening of the edges of the mantle is owing to the pre sence of a great quantity of muscular fibres, fre quently to several rows of contractile tentacular cilia (m, m, fig. 347; e, ,fig. 361 & 362); and, lastly, to that of an organ, which is the secerning apparatus of the shell. The muscular fibres are distributed some to the edges of the mantle, and others to the tentacula with which it is fringed. The whole of these parts are extremely retractile, and are endowed with such sensi bility that the slightest contact is perceived, as is evinced by their instantaneous contraction.