MOLLUSKS. 319 
ming; and in some produces the byssus, a collection of 
threads by which the animal attaches itself to rocks and 
other objects. In most of those which inhabit wnivalve 
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- 
ush tribe we have the most efficient means of locomotion 
in the shape of arms, and in some of this group there are 
fin-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 (cepady, kephale, head), those which 
have heads; and the Acephalous, 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 
swim 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 this 
