_CRUSTACEANS 247 
one contains three long teeth which meet laterally and grind the 
food ; this is known as the gastric mill. 
When sufficiently fine, the food passes through a strainer of 
stiff bristles into the smaller portion of the stomach, where it is 
partially digested, and from there enters a long, straight intes- 
tine which reaches the length of the body and opens to the out- 
side on the under side of the telson, or last segment. A large 
liver also pours its secretions into the intestine. The green sub- 
stance commonly called “fat” in lobsters is the liver. 
The heart consists of an elongated tube, or a short sac, which 
lies directly under the integument of the back. From this heart- 
sac, blood, which is colorless, is sent by arteries to all parts of the 
body ; it then collects in spaces called venous sinuses, from which 
it goes to the gills, and thence back to the heart. 
The nervous system begins in a large ganglion in front of the 
mouth, called the brain ; from this two branches arise, which pass on 
each side of the digestive organs, meeting in ganglia in each seg- 
ment and extending the whole length of the body. The gills, by 
which the animal breathes, are upon the limbs, or on the walls of 
the body immediately adjacent to them, and are generally inclosed 
in special chambers. In lobsters and crabs two such chambers 
are found under the flaps of the carapace, above the walking-legs. 
Gills are divided so as to present much surface to the water, from 
which they absorb oxygen. They are like a dense mass of little 
tubes arranged along acentral tube. The class has two kinds of 
eyes, simple and compound ; the latter are composed of a number of 
eyes. In some species the eyes are placed on the ends of mova- 
ble stalks, which enable the creature to see in all directions and 
from a higher plane than the body occupies. As arule, the eyes 
occur in the head region, but in the shrimp Huphausia they are on 
the thorax and abdomen. In barnacles simple eyes exist in the 
young stage, but in adult forms there are no apparent visual organs. 
The ear (so called) consists of a sac containing small silicious 
particles suspended in fluid. Numerous fine hairs on the inner 
surface of the sac connect with nerve-fibrils. 
The organs of hearing are in various places. In decapods, or 
the larger Crustacea, they are at the base of the antennules. 
