CRU8TACBA. 207 



basal joint, bearing two jointed branches, the exopodite 

 and endopodite. With growth of the animal the exopo- 

 dite frequently disappears. 



The gills by which most Crustacea breathe are thin out- 

 growths of the body, usually closely connected with some of 

 the appendages, either of the thorax or of the abdomen. In 

 shape they may be plates or plumes or sacs, but all are trav- 

 ersed by blood-vessels so that the blood is brought in close 

 proximity to the water. In some cases these gills hang 

 freely into the water, in others they are placed in special 

 gill-chambers, and then there is an arrangement of parts 

 for pumping fresh water over them. In the terrestrial 

 Crustacea these gills still serve as breathing-organs, as in 

 the sow-bugs, and are constantly kept moist. In some of 

 the lower Crustacea there are no special organs of respira- 

 tion, the thin walls of the body affording sufficient surface 

 for the purpose. 



The alimentary canal is nearly straight, and there is usu- 

 ally a chewing-stomach in which the food is ground by hard 

 teeth in the walls, and beyond this there is frequently a 

 straining-stomach. A large so-called liver is always present, 

 pouring digestive juices into the alimentary canal behind 

 the stomach. The eyes are either simple or compound. In 

 the simple eyes there is a single lens for the whole structure, 

 while the compound eyes are composed of many separate 

 eyes, each with its own lens. In some cases these eyes are 

 placed on jointed stalks, at others they are in the walls of 

 the head. Ears have been found in some forms. Usually 

 they are sacs in the base of the antennulae, but in the opos- 

 sum-shrimps they occur near the end of the abdomen. The 

 hairs which occur over the body are organs of touch, and 

 possibly some of them serve as organs of taste and smell as 

 well. 



