318 NATURAL HISTORY. 
with the growth ofthe 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 0 back and front views of the shell of a 
0 Ment 
Mpa 
SMe 
\ 
a4 aon ae Sie 
NU o Ais 
ul 
AES Aw 
FICHE 
wa : 
i! pl a | 
(aN 
SS 
young Mollusk, and at ¢ 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 dévalve shells there is a 
fleshy, tongue-like projection called a foot, 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- 
