52 APPARATUS OF MOTION. 



is too small for this purpose, and in some it exists only at 

 a very early period, and is lost as the animal is de- 

 veloped, 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 gelati- 

 nous or transparent, and what is very curious, the tissues 

 are the same as those of woody fibre, as for example, in the 

 Ascidia. As a general thing, these solid parts do not aid in 

 locomotion, so that the mollusksare 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 as 

 oars, in swimming. 



156. The muscles of mollusks either form a flat disc, 

 or are distributed 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 of the muscles 

 may be, they always form very considerable masses, in pro- 

 portion to the size of the animal, and have a soft and mu- 

 cous appearance, such as is not seen in the contractile fibres 

 of the other departments of the Animal Kingdom. This pe- 

 culiar aspect no doubt arises from the numerous small 

 cavities found in the muscles, and the mucous glands which 

 are distributed through them. 



157. In the Articulated animals, the solid parts are rings, 

 generally of a horny structure, but sometimes calcareous, 

 and successively fitting into each other. The tail of a 

 lobster gives a good idea of this structure, which differs, in 

 the several classes of this department, merely as to volume, 

 form, solidity, number of pieces, and the degree of motion 

 which one ring has upon another. In some groups the 

 rings are consolidated, so as to form a shield or carapace, 

 such as we see in the crabs. In others, the rings are so 

 soft that the body is capable of changing into every possi- 

 ble form, as in the leeches and worms generally. 



