420 CHARACTERS OF INVERTEBRATE ANIMALS 



of an adductor muscle and opened by the action of an elastic 

 ligament. The body of the animal is extremely short, and the 

 abdomen is a mere vestige. Segmentation is only indicated by 

 the appendages, of which there are but seven pairs, five belonging 

 to the head (two pairs of feelers and three pairs of jaws), and the 

 remainder, in the form of narrow pointed legs, to the thorax. 

 These legs can be protruded from the shell, and so can the well- 

 developed feelers which are used as organs of locomotion, as in 

 a nauplius larva. There is an unpaired eye on the front of the 

 head. 



Order 3. — FORK-FOOTED CRUSTACEA (Copepoda) 



This order is a vast assemblage of species which are for 

 the most part minute, and occur in all parts of the world both 

 in salt and fresh water. Many of them are found in huge shoals 

 at the surface of the open sea, forming a variety of plankton, as 

 such assemblages are termed, which furnishes an important item 

 in the food of whales and of many fishes, such as the herring. 

 The fishes, however, do not have it all their own way with the 

 Copepods, for attached to their eyes and gills may often be found 

 parasitic members of the order, which are often strangely modified. 

 Leaving such forms out of consideration for the present, and 

 turning our attention to free-living Copepods, we may take a 

 common fresh-water genus, Cyclops (fig. 260), as a type of the 

 order. It can be distinguished with the naked eye as an active 

 whitish creature with a jerky mode of progression. 



The body of Cyclops has not unaptly been compared in shape 

 to half of a split pear, with the convex side dorsal and the stalk 

 corresponding to a tail. The body is distinctly segmented, and 

 the five head-segments are fused with one another and with the 

 first thoracic segment. Then follow the five free segments of 

 the thorax and the four narrow segments of the tail, the last of 

 which bears a tail-fork provided with two bunches of bristles. 

 Upon the front of the head is a single reddish eye, which has 

 suggested the generic name of Cyclops; and there are the usual 

 five pairs of head appendages, of which the first, i.e. the antennee, 

 are very large and employed as oars. The thorax bears four 

 large pairs of forked swimming- feet, and the abdomen is limbless. 

 The female, which is the sex commonly seen, usually has a pair 

 of large egg-sacs attached at the base of the abdomen. 



