794 FRESH-WATER BIOLOGY 



different shapes in the sexes. In the female they are conical 

 (Fig. 1245 a'^), while with the male (Fig. 1299 e) they are con- 

 verted in a peculiar manner into powerful prehensile organs which 

 serve for grasping the female during copulation. The palps of 

 the right and left sides in the male are different in size and shape 

 (Fig. 1246 e-f). The form of these palps is regarded as of specific 

 importance. 



The two pairs of antennae are found in the head region, and in 

 most cases are provided with long natatory setae, which aid in 

 swimming (Fig. 1268^). The mandibles and first maxillae serve 

 as mouth parts. The fifth pair may be modified in some cases, as 

 in the Limnicythere, serving as legs — in most cases as maxillipeds 

 or second maxillae. The fifth pair is known as the first legs 

 (Fig. 1260 J), and the sixth pair is known as the second legs (Fig. 

 1285 d). The second legs are commonly not ambulatory, but are 

 bent backwards within the shell. They are often called the 

 *' cleaning feet" on account of their observed use in cleaning the 

 valves of any foreign matter. The mouth parts commonly carry 

 a number of setae which create a current of water between the 

 valves for respiratory purposes. 



The general color of the surroundings seems to have some rela- 

 tionship to the general color of the forms present. For instance, 

 all those living in algae-rich habitats are notably green, as many 

 species of Cypris, while those creeping about on the bottom amongst 

 dead leaves and ooze, are commonly devoid of any especial pig- 

 ment, as are most species of Candona. The color of the various 

 forms varies from yellowish white to yellow, green, blue, and violet 

 to purple. The species of Candona are commonly of a pearly 

 to yellowish white, while Cypris, Cypridopsis, and Cypria — forms 

 that inhabit algae-rich regions — commonly show a greenish color. 



The food canal begins with a mouth, which is bounded by upper 

 and lower lips. It is interesting here to observe that the marine 

 forms belonging to the genus Pyrocypris are provided with phos- 

 phorescent glands in the upper lip, which cause much of the 

 phosphorescence of the sea. From the mouth the food passes 

 through a short esophagus to a stomach, which is commonly 

 followed by a short constriction separating it from the short 



