THE MUSEUM 
OF 
NATURAL HISTORY. 
ZOOLOGY. 
Sub-kingdom — MOLLUSC A* — Molluscous Animals. 
If we follow the arrangement of the animal kingdom, 
as exhibited by the immortal Cuvier, we find that 
animals are constructed upon four different types, which 
have been called Suh -kingdoms. The first of these is 
the sub -kingdom Vertebrata, or vertebrated animals, 
and has occupied the former part of this work. The 
second sub-kingdom is that of the Mollusca, composed 
of those animals that are generally known by the name 
of Shells or Shell-fish. Formerly the animals which 
formed and inhabited shells were little known, and by 
those who studied this interesting branch of natural 
history, were very little attended to. The shells or 
coverings of the animals alone attracted the attention 
of the student, and hence the study of this branch of 
natural science received the name of Conchologyf — an 
appellation still in general use. At the present day, 
however, the animals receive an equal share of atten- 
tion from the naturalist ; and the arrangement of the 
hard persistent coverings, or shells, of these animals, 
depends almost entirely upon the structure of the soft 
animals which form them. We find, too, that a large 
number of animals which are true shell-fish, so to 
speak, have no shells, and hence the name of Mollusca 
now adopted to contain all, both shell-bearing and 
shell-less species. The Greeks, as Aristotle and his 
followers, called them by a similar name, Malakia 
(fioiXaTiia.), and the study was hence by several natur- 
alists called Malacology. 
Molluscs may be defined as animals possessing a 
body which is soft and fleshy, devoid of bones or any 
internal skeleton, and not divided, as in insects and 
worms, into rings or articulations. This body is 
covered with a soft, initable, and muscular skin, which 
is moistened by a viscid liquor that exudes from it, 
and which is in very many instances of sufficient extent 
to form fleshy folds that envelope, more or less com- 
pletely, the whole body as in a mantle or cloak. In 
• From the Latin word mollis, soft. 
t From the Greek words xi>yx''ii ^ shell, and Xoyo;. a dis- 
course. 
some cases this skin is naked, and then the mantle is 
thick and mucous ; in the greater number of the species, 
however, it is protected by a hard, persistent, calcare- 
ous covering called a shell, and then this mantle is 
thin and transparent. The animal, in the groat majority 
of species, is of an elongate form, walks upon a central 
disc or foot, and is provided with one or more pairs of 
organs on- the head and sides. The principal organs 
of the body are in pairs and are symmetrical, thus dis- 
tinguishing them from the Radiated sub-kingdom here- 
after to be mentioned. But the most essential character 
lies in the arrangement of their nervous system. This 
system consists of a certain number of enlargements 
or centres, called ganglia, from which the nerves are 
given off to the different parts of the body. These 
ganglia are principally concentrated around the entrance 
Fig. 209. 
to the alimentary canal, and form a collar or ring 
w’hich surrounds the (esophagus or throat, and is con- 
nected with other ganglia disposed without symmetry 
among the viscera, or in the neighbourhood of the 
organs of locomotion — fig. 209. The ganglia “ on the 
sides and upper part of the ring represent the brain, 
and supply nerves to the eyes, tentacles, and mouth; 
