704 



CONCHIFERA. 



also occur; these can be aptly enough alluded 

 to in the anatomical description. 



From 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 Mullusca subidentia. 

 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 system. 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, Testacea Utriusque Sici- 

 ii(E ; 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 lacteal vessels. In a very 

 interesting memoir, Mangili exposed the error 

 which Poli had committed, and rectified it by 

 assigning to the vasa lacl.ea 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 Dimyaria, 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 oesophagus, 

 towards the base of the labial palps (1,1, 

 fig. 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, fig. 360) running across or over the oeso- 

 phagus. From the edges of the ganglions 

 many 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 



Fig. 360. 



Nervous system of an Unio. 



the mantle, and detach numerous branches. 

 From the posterior edges of these anterior 

 ganglions there is one, and occasionally there 

 are two nervous branches of considerable size 

 sent off (4, 4, Jig. 360); these descend along 

 the body towards the base of the branchia?, 

 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 

 branchiae. 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 netvous 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 terminates 

 at the point of commissure in a small ganglion. 

 This little ganglion is not met with in the 

 Dimyaria without a siphon ; neither does it 

 appear in the Monomyaria. When the siphons 

 occur, however, a retractor muscle, peculiar to 

 them, is almost invariably found also, as we 

 have already seen. When these two parts 



