MOLLUSCA. 
throughout its whole length. In many species there is a Mollusca. 
Mollusca. THE Molluscous animals to which our’attention is to be di- 
head, not, however, analogous to that member in the yer- ~~\——/ 
> rected in this article, formed, for a long period, two distinct 
objects of study. The naked species were considered as 
constituting a particular group, to which the term Mottus- 
ca was in a great measure restricted; whilst the shelly spe- 
cies attracted a large share of attention, being known as 
objects of study by the term Concnorocy. Of late years, 
however, this kind of systematical separation of the naked and 
shelly species has been supported by few advocates, whilst 
those who have urged the propriety of their union have not 
only been numerous, but acknowledged as the intelligent 
and successful investigators of nature. In consequence of 
these changes, which have taken place in public opinion by 
the introduction of more correct notions of the principles of 
classification, we have in this edition of our work, resolved 
cn incorporating the articles Motiusca and ConcwoLoey, 
suppressing the latter term as unnecessary. Indeed, in a 
physiological point of view, there is no definite boundary 
between the naked and the shelly species, many of the re- 
puted naked species having shells concealed under their 
integuments, while in many of the shelly species, the solid 
plate or shell can only be reached by removing the sur- 
rounding soft or fleshy matter. But the propriety of aban- 
doning the old divisions will become sufficiently obvious, 
when we consider that all the species belong to one group 
in the INVERTEBRATE class of animals; that the organs of 
the species have many points of resemblance; and that al- 
though differences prevail in the character of the protecting 
covering, the gradations from one condition of the integu- 
ments to another, are too minute, in many cases, to admit 
of precise limits being fixed. We shall therefore consider 
shells as peculiar modifications of the external coverings of 
certain species, and, in this subordinate character, incorpo- 
rate the naked and shelly species; having recourse to the 
integuments, however, in connection with the other organs, 
in the distribution of the genera and other divisions of the 
class now denominated Moxuusca. 
In treating of this extensive division of the invertebrate 
animals, we shall confine our remarks to a general view of 
their physiology, taking notice of the peculiarities of their 
forms, organs, and functions. This will prepare us for a 
condensed view of the progress of the science, in reference 
to systematical arrangement, and the illustration of the cha- 
racters of those divisions or groups into which the species 
may be distributed, whether for assisting the student in his 
labours of investigation, or for facilitating the judicious dis- 
position of the established truths of the science. We shall 
then advert, in the last place, to molluscous animals as ob- 
jects of utility. The limits of the article will unavoidably 
restrict us to a very brief exposition of the various subjects 
connected with this very extensive department of natural 
history. 
CHAP. I.—PHYSIOLOGY. 
Physiology Molluscous animals exhibit very remarkable differences, 
both in their form and in the number and position of their 
external members. Neither head nor foot can be observed 
in some species; the principal organs being enclosed in a 
bag pierced with apertures for the entrance of the food, and 
egress of the excrementitious matter. In others, with an 
exterior still remarkably simple, cuticular elongations, term- 
ed Tentacula, surround the mouth, and a foot, or instrument 
of motion, may likewise be perceived. This last organ is in 
some free at one extremity, in others attached to the body 
tebral animals, and containing the brain and organs of the 
senses, but distinguished merely as the anterior extremity 
of the body, separated from the back by a slight groove, and 
containing the mouth and ¢entacula. 
In many of the animals of this division, the different mem- 
bers of the body are in pairs, and are arranged, in reference 
to a mesial plane, into right and left. In some, a part only 
of the organs has respect to a mesial plane, other parts being 
single, or in unequal numbers. In other species, the or- 
gans, which are not in pairs, are arranged round a central 
axis, and give to the external form a radiated appearance., 
But these characters are exceedingly variable and uncertain, 
as indicating the limits of particular tribes; since, in differ- 
ent parts of the same animal, modifications of all these forms 
may be readily distinguished. 
The skin ofmolluscous animals is more simple in its struc- Skin. 
ture, than the same organ in the vertebral animals. The 
cuticle is here very distinct ; and, as in other classes, it is 
thick and coarse where much exposed, but thin and delicate 
in its texture, where it lines the internal cavities. A mu- 
cous web may be detected in the cuttle fish and slug, but of 
great tenuity. The cordwm is destitute of a villous surface; 
and on its central aspect it is so intimately united to cellu- 
lar substance, that its fibrous structure can scarcely be dis- 
tinguished. The muscular web may, in general, be readily 
perceived. Its fibres proceed in various directions, accord- 
ing to the kind of motion to be executed, and extend or cor- 
rugate the skin at pleasure. 
The appendices of the skin in this class of animals ought 
to be carefully studied, as they furnish the most obvious 
marks for distinguishing species, and for constructing divi- 
sions in their systematical arrangement. The appendices 
of the cuticle are few in number, and perhaps ought to be 
considered as limited to hairs. These, in some species, in- 
vest the surface regularly and closely, and may be observed 
on those which live on land, as well as those which reside 
in water. In some cases the hairs may be considered as 
united, and forming continuous crusts or ridges. These 
hairs, as well as the cuticle, are liable to be worn off, and in 
some places can seldom be perceived, unless in early age. 
The most important appendix to the skin, for such it must Shell. 
be considered, appears to be the shell. This part is easily 
preserved, exhibits fine forms and beautiful colours, and has 
long occupied the attention-of the conchologist. The mat- 
ter of the shell is secreted by the corium, and the form which 
it assumes is regulated by the body of the animal. It is 
coeval with the existence of the animal, and appears previ- 
ous to the exclusion from the egg; nor can it be dispensed 
with during the continuance of existence. The solid mat- 
ter of the shell consists of carbonate of lime, united with a 
small portion of animal matter, resembling coagulated al- 
bumen. 
The mouth of the shell is extended by the application of 
fresh layers of the shelly matter to the margin, and its thick- 
ness is increased by a coating on the inner surface. These 
statements are abundantly confirmed by the observations of 
Reaumur, (Mémoires de 7 Académie des Sciences, 1709), 
whose accurate experiments have greatly contributed to the 
elucidation of conchology. Ifa hole be made in the shell 
of a snail, and a piece of skin so glued to the inner margin 
as to cover the opening, the shelly matter will not ooze out 
from the broken margin of the fracture, and cover the out- 
side of the skin, but will form a coating on its inner surface, 
