40 THE LIFE OF CRUSTACEA 



The subclass Copepoda comprises animals, for 

 the most part of microscopic size, which are abundant 

 in fresh water and in the sea. The common fresh- 

 water genus Cyclops (Fig. 14) furnishes a good 

 example of the type of structure characteristic of 

 the class. The body is somewhat pear-shaped, with 

 a narrow abdomen ending in a '* caudal fork." The 

 body is divided into somites, and there is no over- 

 lapping carapace, although the head and the first 

 two thoracic somites are coalesced. There are four 

 pairs of two-branched, oar-like, swimming feet, and a 

 fifth pair, found in some other Copepoda, is repre- 

 sented in Cyclops by minute vestiges on the first 

 segment of the narrow posterior part of the body. 

 The antennules are very large, unbranched and com- 

 posed of numerous segments ; the antennae are 

 much sm.aller. In addition to the usual mandibles, 

 maxillulae, and maxillae, there is a pair of maxillipeds 

 which really represent the first pair of trunk limbs. 

 There is a single red eye in the middle of the front 

 of the head. This eye is not formed, like the single 

 eye of the Cladocera, by fusion of a pair of eyes, but 

 it corresponds to a median eye of simple structure 

 which is found in the Branchiopoda, Ostracoda, and 

 many other Crustacea, in addition to the paired 

 compound eyes. From the fact that this median 

 eye is the only one present in the earliest larval 

 stage of Crustacea, the Nauplius (see Chapter IV.), 

 it is sometimes known as the " nauplius eye." The 



