32 NATURAL HISTORY OF ARTHROPODS. 



by the long plumose hairs with which they are covered. The style of motion of the 

 various species of the Cladocera largely depends upon the size and mode of handling 

 these organs. In the Daphnidae, where they are large, the animal usually moves in a 

 somewhat dignified manner by the slow strokes of the antennae; in the Lynceidae, 

 where they are small and moved by powerful muscles, the motion is either a hurried, 

 toddling scramble, curiously suggestive of indecision, or, in the smaller and more 

 active forms, it becomes a rapid and decisive darting about. Each genus is charac- 

 terized by its own style of locomotion, and the motion is often varied with the species, 

 so that the experienced observer can frequently identify the forms in an aquarium with 

 the naked eye, even when but a fiftieth or even a hundredth of an inch in length. 



Resuming our anatomical account, the head also bears the antennulae, fringed with 

 sense-hairs. These organs are brought down to the edges of the valves, so that they 

 may take cognizance of whatever passes into the shell cavity. The large shell is but 

 an enormous development of the integument of the back of the head, and consists of 

 two layers, between which there is provision for an extensive circulation of the blood. 

 The upper portion of the shell cavity serves as a brood cavity, while the adjacent 

 portion of the valves develops the ephippium, to be described further on. The shell 

 is marked with lines and dots, the pattern varying with the species. 



The slender body terminates in an abdomen which is bent forward and ends in two 

 claws, and is further provided with a row of spines on each side. This terminal joint 

 is very useful to the animal ; it keeps the valves free from extraneous matter, kicking 

 out any unwelcome particle which may have obtained entrance, and serves as a 

 powerful lever in case the animal be caught. To the under side of the body are 

 attached five pairs of legs, all completely covered by the shell. These are broad, flat, 

 and leaf-like, composed of several lobes and furnished with numerous hairs. The 

 largest lobe of the first four pairs is scoop-shaped, with the convexity forward, and 

 with the outer edge thickly set with long hairs, which reach to the inner surface of the 

 valves. These legs move together, backward and forward, and so keep up a constant 

 current of water through the valves, which serves for respiration, and also brings the 

 food. The fifth pair of legs is of a different shape, and is attached to each other 

 along the middle line, and flap like a pair of wings, but in reverse direction to the other 

 legs. The use of this arrangement will appear when the mouth-parts have been 

 described. 



The mouth lies just within the edge of the valves. It is guarded below by the 

 long, tongue-shaped lower lip, which is normally pressed up against the lower side of 

 the body. Here are the two mandibles, whose grinding surfaces are rubbed together 

 to triturate the food. Just behind them are a pair of maxillae, short, hand-shaped 

 organs, which serve to thrust the food between the mandibles. 



The mode of obtaining the food is as follows : The first four pairs of legs flap 

 vigorously, and propel a current of water through the shell cavity from front to rear. 

 Just at the edge of the valves lie the small antennae, which take notice of any large or 

 unwelcome particle. When such an intruder is perceived, the valves close and the 

 motion of the legs stops. The fifth pair of legs, working in a direction opposite to that 

 of the others, creates a sort of whirl in the water, and directs a current upward, which 

 passes between the legs forward to the mouth. The particles of food are thus stored 

 up in this space until the mouth is ready for them, when the lower lip is depressed, the 

 mouth opens, and the maxillae push a quantity of food between the mandibles. In a good 

 locality a large amount of food will be seen as a dark mass between the legs awaiting 



