MOLLUSCA, 
In comparison with the great variety of form exhibited by the Vertebrata, the | 
Mollusca appear but poorly represented. Their form, such as it is, is always more 
or less lumpy and sometimes even almost shapeless, there being in many cases no 
articulation of the body and no definitely formed limbs, a distinct head even being 
absent in some instances. Their most prominently conspicuous part, the shell, can | 
scarcely be reckoned as a part of the body, inasmuch as it is only an appendage 
to it, and does not partake of its vitality. 
Their very name, derived from the latin Mollis, Soft, indicates the character of 
their body — a soft mass, supported neither by an internal bony skeleton, 
as in the Vertebrata, nor by an external one, as seen in the shells of the Crustacea. 
The body on the contrary is enveloped in a soft and highly distensible skin, known | 
as the ‘Mantle’, which, in those genera that are furnished with a shell, secretes the 
calcareous matter which goes to form the same. Nothing need be said therefore as 
to the build or outward appearance of the body, as these are continually fluctuating | 
with every change of position assumed by the creature. In the more highly deve- 
| loped types, such as the cephalopods and snails, the body has to a certain extent a 
definite shape, the head also being more or less distinct. The cephalopods are fur- 
nished with two large glaring eyes, while in most of the snail tribe organs of 
sight are present, they being placed at the termination of the spindle-shaped feelers 
or horns. In the lower grades on the other hand every indication of a distinct head 
is wanting, while in most of them there are no organs of vision to be detected. 
In the internal economy of the body, the most prominent feature consists of 
the organs of digestion. The liver and salivary glands and other organs forming 
part of the digestive apparatus are all highly developed, while those parts of the 
body which serve to seize and secure their prey are also formed in a manner ade- 
quate to fulfil those functions. In the snails the mouth is armed with a peculiar 
organ of mastication resembling a rasp in its action; the mussels on the contrary 
are not provided with anything of this nature, in lieu of which however the whole | 
of the interior surface of the mantle is beset with a fine hair-like fringe which is 
kept in continual motion, and thus serves to convey food to the mouth. The circu- | 
lation of the blood is effected by means of a heart, furnished with ventricle, auricle | 
and pericardium. In the land-snails and most of the fresh-water snails respiration | 
is carried on by means of lungs; the cephalopods are furnished with gills on the 
contrary, these being sometimes two and sometimes four in number, while the 
mussels are endowed with a double pair of gills. 
Some of the Mollusca feed on animal, others on vegetable substances; all of | 
them being excessively voracious, especially the cephalopods, which lay a vast | 
number of fishes and other inhabitants of the deep under contribution to afford | 
them sustenance. The terrestrial and fresh-water snails mostly live upon vegetable | 
matter, which they grind up with their rasp-like tongues; the marine snails and | 
other descriptions of Mollusca which inhabit the sea feed both on animal and vege-— 
table substances, those which are confined to the former description of food being | 
furnished with peculiar suctorial organs to enable them to extract nutriment there- 
from. In conclusion, those mussels which are incapable of masticating their food are 
ng! ea to restrict themselves to the smallest atoms of organic matter to be found | 
in the water, whether animal or vegetable, which they seize by means of their 
fringed mantle. 
