CRUSTACEA 331 



and bottom-living forms. Parthenogenesis is common among them, 

 and in some males have never been found. 



Cypris (Fig. 237) is a common British freshwater genus. It 

 swims well, by means of its antennae, but is not pelagic. The current 

 which sweeps the food into the shell is set up mainly by the action of 

 the epipodites of the maxillules (whose fan of setae is conspicuous in 

 the figure), and the food particles are gathered from the current by 

 long bristles on the palps of the mandibles and passed towards the 

 mouth by the endites and endopodites of the maxillules, assisted by 

 the gnathobase of the maxillae. The first trunk limb is used in crawl- 

 ing, and the second in cleaning. Cypris lacks the compound eyes and 

 the heart, which are found in some other members of the class. 



Class COPEPODA 



Free or parasitic Crustacea, without compound eyes or carapace; 

 with biramous or uniramous palp, or with none, on the mandible; 

 and typically with six pairs of trunk limbs, of which the first is always 

 and the sixth often uniramous, the rest biramous, and none are 

 situated behind the genital aperture (i.e. on the abdomen). 



The form of the body varies greatly in the members of this class, 

 from the pear-shaped or club-shaped free-swimming genera to the 

 distorted, unsegmented, and sometimes even limbless adults of some 

 of the parasites. In all cases in which the segmentation is complete 

 the number of somites is the same — sixteen, including a preantennu- 

 lary somite but not the telson — throughout the group, but the actual 

 tagmata, which do not conform to the limits of the head, thorax, and 

 abdomen, are not uniform in all members of the class. 



We shall take as an example of the group the little freshwater 

 crustacean Cyclops (Fig. 238) which, though it is not one of the 

 most primitive members of the Copepoda, is well segmented and can 

 be obtained everywhere in ponds and ditches. The ^^ope of this animal 

 is that of a slender pear with a stalk. The front part of the pear is un- 

 segmented; this is a compound head or "cephalothorax", composed 

 of the true head and the first two thoracic somites : beneath, in front, 

 it bears a blunt projection, the rostrum. The rest of the broad part 

 of the body contains three somites, the third to fifth of the thorax. 

 The cephalothorax and these free thoracic somites are produced at 

 the sides into low pleural folds. The stalk begins with a short somite 

 which is united to, but distinguishable from, that which succeeds it. 

 The next somite bears the genital openings and is therefore, on the 

 convention we have adopted (p. 296), the last somite of the true 

 thorax, but is usually reckoned as the first of the abdomen; in the 

 female it is fused with the somite which succeeds it. Two free ab- 



