76 APPARATUS OF MOTION. 



and is lost as the animal is developed, so that at last there is 

 no other covering than a slimy skin. In others, the skin 

 becomes so thick and firm as to have the consistence of 

 elastic leather ; or it is gelatinous or transparent, and, what is 

 very curious, these tissues may be the same as those of woody 

 fibre, as, for example, in the Ascidia. As a general thing, 

 the solid parts do not aid in locomotion, so that the mol- 

 lusks are mostly sluggish animals. It is only in a few rare 

 cases that the shell becomes a true lever, as in the Scollops, 

 (Pecten,) which use their shells to propel themselves in 

 swimming. 



156. The muscles of mollusks either form a flat disk un- 

 der the body, or large bundles across its mass, or are dis- 

 tributed in the skin so as to dilate and contract it, or are 

 arranged about the mouth and tentacles, which they put 

 in motion. However varied the disposition may be, they 

 always form very considerable masses, in proportion to the 

 size of the body, and have a soft and mucous appearance, 

 such as is not seen in the contractile fibres of other animals. 

 This peculiar aspect no doubt arises from the numerous 

 small cavities extending between the muscles, and the secre- 

 tion of mucus which takes place in them. 



157. In the Articulated animals, the solid parts are ex- 

 ternal, in the form of rings, generally of a horny structure, 

 but sometimes calcareous, and successively fitting into each 

 other at their edges. The tail of a lobster gives a good 

 idea of this structure. The rings differ in the several 

 classes of this department, merely as to volume, form, solid- 

 ity, number of pieces, and the degree of motion which one 

 has upon another. In some groups they are consolidated, so 

 as to form a shield or carapace, such as we see in the 

 crabs. In others, they are membranous, and the body is 

 capable of assuming various forms, as in the leeches and 

 worms generally 



