CRUSTACEA. 219 



mere stump, as in the barnacles. Any of the segments 

 except the last one may bear appendages. The append- 

 ages most commonly present are the two pairs of antennae 

 in front of the mouth, next those concerned in eating and 

 grouped as 'mouth-parts.' Of these there are always a 

 pair of biting-jaws or mandibles, then two pairs of acces- 

 sory jaws or maxillae, and lastly a varying number of 

 * jaw-feet' or maxillipeds. Behind these there may be the 

 walking-feet upon the thorax and the swimming-feet upon 

 the abdomen. 



If these appendages be studied in the adult of some 

 species or in the young of all, they are found each to con- 

 sist of a basal joint, bearing two jointed branches, the 

 exopod and endopod. With growth of the animal the 

 exopod 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 

 traversed 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 ter- 

 restrial 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 respiration, the thin walls of the body affording suffi- 

 cient surface for the purpose. 



The alimentary canal is nearly straight, and there is 

 usually a chewing-stomach in which the food is ground 

 by hard teeth in the walls, and beyond this there is fre- 

 quently a straining-stomach. A large so-called liver is 



