MOIiUSKS. 319 



ming ; and in some produces the byssus, a collection of 

 threads by which the animal attaches itself to rocks and 

 other oV)jects. In most of those which inhabit univalve 

 shells there is no projecting foot ; but the under side of 

 the mantle is thickened into a fleshy disk, which by its 

 contractions and expansions effects the progression of the 

 animal, as is seen in the common Snail. Among the Mol- 

 lusks similar to these in structure, but having no shell, the 

 whole mantle is muscular, enabling them to move quite 

 freely, especially those that live in water. In the Cuttle- 

 fish tribe we have the most efficient means of locomotion 

 in the shape of arms, and in some of this group there are 

 iin-like appendages, the arms being quite short. 



548. Leading such a sluggish life as most of the Mol- 

 lusks do, their destiny seems to be to grow, by their di- 

 gestive powers, into a well-fatted mass, so that they may 

 be good food for other animals that inhabit the deep, and 

 some of them for man. 



549. Almost all of these animals breathe by gills; but 

 some, like the Snails and Slugs, have something like lungs, 

 as they live in air. The blood is nearly colorless, and 

 circulates in a regular system of arteries and veins con- 

 nected with a heart. 



550. This sub-kingdom has two grand divisions — the 

 Cephalous Mollusca (ce^aXi), Jcephcde, head), those which 

 have heads ; and the Acepiialous, those which are head- 

 less. I will first speak of the Cephalous. All belonging 

 to this division that have shells have those which are 

 univalve. The Cephalous Mollusks are divided into three 

 groups : 1. Cephalopoda, those which have feet arranged 

 in a circular manner around the head. 2. Pteropoda, 

 wing-footed. These have a pair of wing-like expansions 

 of the mantle, which serve as fins, and enable them to 

 s^vim quite rapidly. This is a small class, but a very in- 

 teresting one. 3. Gasteropoda, belly-footed. These have 

 a single broad foot on the under surface of the body. 

 The first two classes belong entirely to the sea ; . but^thi^ 



02 



