104 THE SKELETON OF MOLLTJSCA. 



parts which they cover. These shells are generally so con- 

 structed as to afford complete protection to the animal within 

 their cavities. In a few, the shell is too small for this pur- 

 pose ; in others it exists only at a very early period, and is 

 lost as the animal is developed, so that at last there is no 

 other covering than a slimy skin. In some the tegumentary 

 membrane 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. In general the solid parts 

 do not aid in locomotion, so that the mollusca are mostly slug- 

 gish in their movements. It is only in a few rare cases that 

 the shell becomes a true lever, as in the scallops (Pecteri), 

 which use the valves thereof to propel themselves in swimming. 



[The shells of a great majority of the gasteropoda are uni- 

 valve, and rolled obliquely, in consequence of the unequal de- 

 velopment of the body of the animal. They consequently 

 form a helix or oblique spiral ; sometimes the coil is towards 

 the right, but in general it is towards the left side. Some 

 univalve shells have a patelloid form, and are symmetrical, 

 without being spiral ; and there are various intermediate 

 groups, by which these forms blend into each other. Some 

 of the shells vary very much in form at different stages of their 

 growth, as shown in the beautiful Cypr&acdssis rufa, from the 

 coral reefs of the South Pacific. Fig. 74, a, is the mature 

 form of that shell, with its greatly developed right lip ; and b t 

 the young, or immature form of the same. T. W.] 



221 . The muscles of mollusca either form a flat disc under 

 the body, or large bundles across its mass, or they 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 in their disposition, the muscles 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 the other divisions of the animal 

 kingdom. This peculiar aspect no doubt arises from the nu- 

 merous small cavities extending between the muscles, and the 

 secretion of mucus which takes place in them. 



222. In the articulated animals (fig. 34), the solid parts are 

 external, 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 



