qb & THE GEELONG NATURALIST. 
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Mollusca do not all possess heads, ene entire class being 
‘quite destitute. When they do exist; they are usually provided 
with tentdeula—as in the marginella (A.4.) by which the animals 
feel their way, and which they have the power of easily retracting 
"when danger threatens. They have generally eyes (B.B.), and other 
organs of sense, but they vary much in this respect, some possessing 
sight and smell, while others are devoid of both, and apparently enjoy 
only that of touch. That of feeling and sensibility to pain must be 
slight, or they could never survive the mutilations they sometimes 
undergo, and they have the power of reproducing parts that have been 
injured or even cutoff. That limb, which, from its being an organ of 
motion, has been called a foot (c.), is much more universally found 
amongst mollusca than the head, and is a member of considerable 
importance. It is very muscular and flexible. Large aud strong in 
gakteropoda and in the burrowing kinds of conchifera, and rough in 
those that bore in rocks, but scarcely existing in those attached to 
other substances. They possess the usual organs necessary for the 
support of life; a nervous system; a heart, with its attendant veins 
and arteries, furnished with a cold white or bluish blood; and an 
apparatus for breathing, consisting in those which live in water, of 
bronchial, placed externally or'internally, and so arranged as to 
have a stream of water continually presented to them; and in those 
hich live in air, of organs fit for its respiration. 
Mollusca are found in all parts of the world, and at all depths 
of the sea, € Nach different zone having the tribes of shell-fish 
peculiar to it. Itis by a knowledge of this the geologist can tell 
by the fossil shell at what depth of the sea the particular section of 
the earth he may have under consideration was originally formed. 
The methods of locomotion adapted by these mollusca which 
move from place to place are various. A great number, like the 
-oysters are perfectly stationary, and do not, when once fixed, move 
from the place selected. — The greater number are continually 
moving about; and various and beautiful are the contrivances by 
which they effect this. Some move along the surface of the ground 
or bottom of the sea by means of their foot which they thrust out 
to a distance in advance, and fix it to some solid object, after the 
manner of the common garden snail. Others, using an action the 
reverse of this, push backwards with the foot and thus impel them- 
‘selves forward. And others, still using the foot for the purpose, 
contrive to advance with considerable agility, by coiling it up tightly 
.and suddenly relaxing it, by this means serving as a spring, it urges 
the body to some distance. There are many species of mollusca 
which are unable to move along the rough surface of the ground, 
‘but manage to advance rapidly enough on the smooth marine plants 
