STRUCTURE OF MOLLUSCA. 113 



one possesses 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 fleshy 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 

 muscles (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 



