318 



NATURAL HISTORY. 



with the growth of the animals in them. Sometimes ad- 

 ditions are made to them, entirely altering the figure, so 

 that two animals of different ages really of the same spe- 

 cies would hardly be recognized as such. In Fig. 250 we 

 have at a and b back and front views of the shell of a 



Fig. 250. 



young Mollusk, and at c and d similar views of the shell 

 of the full-grown animal. The addition of the spines 

 bears some analogy to the addition of horns in some of 

 the Mammalia. 



547. Most of the Mollusca can move about but little, 

 and some none at all. They have but little muscle, and 

 are in this respect in striking contrast with the Articu- 

 lata, which are nearly all muscle ( 383). It is only where 

 the body is naked (that is, without a shelly covering), or 

 where a portion of the body can be projected out from 

 the opening in the shell, that any active movements can 

 be effected. In many inhabiting bivalve shells there is a 

 fleshy, tongue-like projection called a/oo, which in some 

 cases enables the animal to leap ; in some is used as a 

 boring apparatus ; in some acts as a sort of fin for swim- 



