Volltext: The Cyclopaedia of Anatomy and Physiology, vol 1: A-Dea (1)

»I=o 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 Mollusca 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 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¬ 
lies; 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 laclea 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, fig. 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, fig. 360) ; these descend along 
the body towards the base of the branchiae, 
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


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