Froin what has now been said it is easy to understand the offices performed by the foot. In. the lithophagous and xilophagous Con chifera, the foot, reduced to its rudimen tary condition, is probably without any par ticular use, unless perhaps it be among the Pholades, where, being in the form of a sucker, it may enable the animal to fix itself to the parietes of the cavity it inhabits. Among the Conchiferous mollusks that live at large, the chief use of the foot is to dig a furrow, into which the animal forces itself partially, and then advances slowly by making slight see saw or balancing motions, a circumstance which has led Poli to designate the whole class of acephala by the title of Iliollusca subsilentia. Several of these Mollusks not only make use of' the foot in the way we have just mentioned, but also employ it as a means of executing sudden and rapid motions, true leaps, by which they are enabled to change their place with great celerity. It is of course unneces sary to say that in those genera whose shell is attached immediately to the bodies at the bot tom of the sea (Chama), the foot is of no use as an organ of locomotion at all events. In the byssiferous species, again, the organ, al though but slightly developed, is the agent in spinning the filaments of this cable.
Nervous systeni.—Anatomists were long ig norant of the existence of a nervous system in the Conchiferous mollusca. Poli first disco vered it in the course of his dissections, whilst preparing subjects for the plates of his magni ficent work, entitled, Testucea Utriusque Sici lia; but he mistook the nervous system, occa sionally of considerable magnitude, for one of absorbent or lymphatic vessels, and spoke of it under the name of /actea/ vessels. In a very interesting memoir, Mangili exposed the error which Poli had committed, and rectified it by assigning to the vasa lactea of his learned countryman their true place as portions of the nervous system.
The acephala have no brain properly so called. The nervous system is symmetrical in the Dirnyaria, but loses this character in some measure in the Monomyaria. This diversity in the nervous system, coinciding with the number of the muscles, gives a higher value to the character which is established on the existence of one or two adductor muscles. In the Dimyaria we find, on each side of the mouth, a small ganglion above the cesophagus, towards the base of the labial palps (1, 1, jig. 360). Each of these ganglions is of an oval or sub-quadrangular shape, and the two are connected by means of a transverse filament (2, jig. 360) running across or over the ceso pliagus. From the edges of the ganglions
niany filaments arise, some of which on the sides descend into the substance of the labial palps (3, Jig. 360); others anterior are distri buted to the edges of the mouth ;. and others run to the lateral parts of' the anterior adductor muscle, gain the thick portion of the edge of the mantle, and detach numerous branches. From the posterior edges of these anterior ganglions there is one, and occasionally there are tvvo nervous branches of considerable size sent off (4,4, .fig. 360); these descend along, the body towards the base of the branchi2e, concealed amidst the visceral mass, and give off filaments in their course to the neighbour ing organs, first to the stomach, then to the liver and heart, and next to the ovary and branchix. A considerable branch descends on each side of the foot, and is expended upon this organ. When the lateral filaments have arrived opposite to the posterior adductor muscle, they advance along its internal sur face, approach one another, and at their point of junction give origin to one or two ganglions of different sizes, but always larger than the anterior ganglions. When the posterior gan glions are some way apart, a nervous filament always connects them. It is from these pos terior ganglions that the nervous cords are detached, the branches of which are distri buted to the whole posterior parts of the ani mal. Some run towards the anus, others to the thin portion of the mantle, and a consi derable number to the thickened margin of the same organ. When the lobes of the mantle are conjoined posteriorly, and are continued from this part by means of siphons, among the nervous branches which follow the thick ened edge of the mantle, one is distinguished of larger size than the others, which terininates at the point of commissure in a small ganglion. This little ganglion is not met with in the Dimyaria w,thout a siphon ; neither does it appear in the Monomyaria. When the siphons occur, hovvever, a retractor muscle, peculiar to them, is almost invariably found also, as we have already seen. When these two parts exist, nervous branches are likewise discovered, destined for them, one for each of the retractor niuscles, and one for each of the siphons. The posterior part of the nervous system of the Dimyaria is so considerable in comparison with the anterior part, that some anatomists have maintained that the title of brain should be given to the posterior ganglions, conceiving them to be of much greater consequence in the organization of these animals, and of more avail in regulating their functions than the anterior ones.