GENERAL IDEAS CONCERNING MOLLUSKS. 163 



The skin forms around the bodies of mollusks an 

 envelope that is usually largely extended at one part, 

 and folded on itself, and this arrangement is called the 

 mantle. 



When we examine a bivalve shell, that of the oyster, 

 for example, we find that each half is formed of a num- 

 ber of layers, which are wider as they are closer to the 

 interior. The external layers are the older, and as the 

 oyster grows larger it extends the internal surface of its 

 dwelling. In the shell of the snail, which is univalve, 

 the effect of growth is to increase the number of turns 

 of the spiral. When the shell is bivalve the two halves 

 are held apart by the action of the ligaments that form 

 the hinge ; they are tightly pressed together by the con- 

 traction of two muscles. The only opening of univalve 

 shells is generally closed by a small mobile disk called 

 the operculum. 



In certain species the shell is so small that it can be 

 of no apparent use. In others there is no external 

 shell, but there is then often an internal calcareous piece 

 analogous to what is called cuttle-fish bone. 



Most of the mollusks have the power of movement ; 

 a large number of them, however, live fixed to rocks and 

 other submerged masses, to which they adhere sometimes 

 by the aid of a fleshy foot, sometimes by filaments that 

 develop on the external surface of the shell. 



The organs that serve the functions of digestion, cir- 

 culation, respiration, and relation, show in the mollusks 

 a great variety of form. Yet certain peculiarities are 

 sufficiently characteristic to be mentioned. 



The digestive tube, composed of a canal having more 

 or less numerous pockets in its course, according to the 

 species, is always arranged somewhat or entirely in the 



