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, whichpass 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 a central 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 a rule, the eyes 

 occur in the head region, but in the shrimp Euphausia 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 silioious 

 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. 



