STRUCTURE OF MOLLUSCA. 113 
one a shell into which it can withdraw its whole 
body for the sake of protection, whilst the other has none; 
and several intermediate forms exist, in which the shell 
bears a larger or smaller proportion to the body, sometimes 
being able to contain nearly the whole of it, and sometimes 
being a mere rudiment, as in the Testacella (fig. 54). 
107. The external form of the body of Mollusks is subject 
to great variation ; and generally has a good deal to do with 
the degree in which the organs of sense and the instruments 
of motion are developed in the particular animal. For these 
are almost always symmetrical, being arranged with equality 
on the two sides of a middle line; whilst the rest of the 
body, containing the organs of nutrition, is often unequal on 
the two sides. But in the lower Mollusca, which have little 
or no power of moving from place to place, even this degree 
of symmetry is altogether lost. Few of the Mollusca have 
any powers of active movement; in fact, the term sluggish- 
‘ness, derived from a characteristic member of the group, very 
well expresses their general habit. The Gasteropods, which 
may be regarded as the types of the whole series, crawl upon 
a fieshy disk, by the successive contractions and relaxations 
‘of which they advance slowly along the surface over which 
they move ; this kind of action is easily studied, by causing 
a Snail or Slug to crawl upon a piece of glass, and by looking 
through this at the under side of its foot. Hence, there is a 
‘great contrast between the inertness of the Mollusca, and the 
high activity of the Articulata. This contrast shows itself in 
the structure of their bodies ; for whilst the chief part of the 
interior of an Insect is made up of the muscles which move 
its legs and wings,—the apparatus of nutrition being small,— 
the chief part of the bulk of a Slug or Snail is given by its 
very complex apparatus for nutrition—there being no other 
Bucks except some small ones connected with the mouth 
‘and head) than the fleshy disk already mentioned. The blood 
‘of the Mollusca is nearly colourless, as it is in the Articulata ; 
‘but the organ by which it is circulated through the body is 
‘much more powerful and complete, bearing more resemblance 
to the heart of Vertebrated animals. The skin is usually 
thick and spongy in its texture ; having muscular fibres inter- 
‘Woven in its substance, so that it can contract or extend itself 
in any part; and having the power of exuding shelly matter 
I 
