THE WATER FLEAS (CLADOCERA) 679 



abdomen is often an efficient aid to locomotion. It ma\- [)u.sh tlu- 

 animal along, as in Ilyocryptus (Fig. mo) and Camptoccrcus (Fig. 

 1119). In Dunhevedia (Fig. 1134) it is peculiarly ciTectivc. broad 

 and stout, covered with numerous small spines and setae, and by ii.s 

 aid the animal may execute sudden and vigorous jumj)s. 



The head also bears the mouth parts: (i) The mandibles (Figs. 

 1050, 1068, 1099, and others); stout, strongly chitinizcd organs. 

 made in one piece and without a palpus. Their op|)osing faces are 

 toothed and ridged and they grind the food very perfectly. (2) The 

 maxillae, a pair of very minute organs, lying concealed on the ven- 

 tral surface of the body, just behind the mandibles. Each is a 

 small, pointed structure, bearing several curved setae. They work 

 like a pair of hands to push the food between the mandibles. (3) 

 The labrum, an unpaired structure, attached to the rear of the 

 head and closing the mouth from below. In many of the Mac- 

 rothricidae and Chydoridae this structure bears a keel or projection 

 which is of systematic value (Figs. 1051, 1060, 1106, 1135). 



The axis of the head may continue that of the body {extended, Fig. 

 1 100), or it may be bent downward {depressed, Fig. 1087). That 

 part in front of the eye is known as the vertex. There is usually a 

 sort of beak in front of, or between, the antennules, which is known 

 as the rostrum, whose size and shape have systematic value. There 

 is commonly a ridge above the insertion of the antenna, which helps 

 to stiffen the side of the head and to support the pull of the antcn- 

 nary muscles. This is the fornix, whose shape and extent may form 

 an important systematic character (Figs. 1063, 1083). 



The shell, though called bivalve, is really in one piece, bent along 

 the back, but never showing a division or joint at this i)lace. It 

 has very different forms, as seen from the side, nearly square, oval, 

 or round. It may be marked in the most various fashions. It 

 may bear hairs, or spines, along the ventral edge. There may be 

 a single spine on the dorsal side, prolonging the junction of the 

 valves, as in Daphnia, or each valve may have one or more spines 

 at the lower posterior part, the infero-posteal angle (Fig. 1076). 

 This angle in the Chydoridae may be acute or rounded, smooth or 

 toothed, and its characters are of systematic value. The shell is 

 always a duplicature of the skin. Its inner wall is far more 



