332 LOCOMOTION OF MOLLUSCA. 



Mollusks ; and it often furnishes characters in classification, 

 which are equally useful with those derived from its form. 



943. The means of locomotion possessed by Mollusca are 

 usually very limited ; and the absence of any constant and 

 regular provision for this function, is an additional evidence how 

 little it enters into the general plan of the group. "We have 

 seen in the Articulated series, that, wherever members or instru- 

 ments for locomotion exist at all, they have the same character 

 and situation, being developed from the several segments of 

 the body, with more or less complete uniformity. Now among 

 the lowest Mollusca we shall find, that some tribes are entirely 

 fixed, during all but the earliest period of their lives ; that in 

 others, although the body is not fixed, it has no other means of 

 movement than that afforded by the respiratory currents, which 

 cause it to advance gently through the water, without (as it 

 would appear) any voluntary control on its own part. In many 

 animals inhabiting Bivalve shells (i. e. Shells which are formed 

 of two parts or valves, united by a hinge) there exists what is 

 termed a foot ; which is nothing else than a fleshy tongue-like 

 projection, sometimes enabling the animal to leap upon hard 

 surfaces, sometimes used as a boring apparatus, sometimes em- 

 ployed as a sort of fin for swimming, and sometimes chiefly 

 useful as the instrument for producing the byssus, a sort of cord 

 by which the animal attaches itself to rocks, &c. In the animals 

 inhabiting the greater number of Univalve shells, there is no 

 such projecting foot ; but the under side of the mantle is thick- 

 ened into a fleshy disc, which, by its contractions and expansions, 

 serves as an instrument of progression. This is well seen in the 

 common Snail. Among the animals allied to these in structure, 

 but not possessing a shell, the whole mantle is muscular ; and 

 by the contractions and expansions of its different parts, the 

 general movement of the body is effected. This is especially the 

 case with the aquatic species of this group ; some of which are 

 thus enabled to swim with considerable rapidity. Again, in 

 the Mollusca of the Cuttle-fish tribe, we find this method 

 replaced by others, which are more efficient, but which are 

 entirely different in their character. This group 13 Distinguished 



