ARTHROPOD. V 



153 



always have a bivalve shell, which may he opened by an elastic 

 ligament on the dorsal side and closed by means of muscles 

 within, passing from one valve to the other. There is a median 

 simple eye and sometimes a pair of compound eyes as well. 

 The number of appendages is seven pairs, live pairs belonging 

 to the head and two to the thorax ; the abdomen is short and 

 without appendages. The hotly is unsegmented and lies com- 

 pletely within the shell. Cypris (Fig. 145) is the most common 

 fresh-water genus. 



Order 3. Copepoda 



The Copepoda (Gr. kouti], oar, and 7rou?, foot) are mostly 

 small, but occur in very great abundance in both fresh and salt 

 water. The body is distinctly segmented except in most para 

 sites and is without a shell. The ,.v 



number of segments is pretty m/ f 



constant; in addition to the head ^IL^Cj*'"" """"N 



segments there are five in the /^?\ nk> \< \ 



thorax and five in the abdomen. \/^x/^^^M^r\ Jh~J 



The appendages are confined to 2 (r- — totP^IS^x^./ 

 the head and the thorax, the Ires. 4 s fQffc^z 



latter having four or five pairs, "^ 



which are biraniOUS. In most FlG.145. Cypris Candida. Lateral aspect ; 

 .1 i • 1 enlarged. 1. antennule; 2, antenna; 



species there is only a simple me- 3 _ ma s ndible . 4 first maxil]a . ., second 

 dian eye. The common water maxilla; 6, first pair of legs; 7, second 



n /- 1 /t— f\ r i • pairof legs; 8, tail; g, eve. (After Zenker, 



flea, Cyclops (Fig. 146), found in fmm shiplev and MacBr ide's Zoology.) 



great numbers in fresh water, is 



a typical representative of the order ; it swims about with a jerky 



motion, which makes it readily distinguishable, and the females 



have a pair of elongated sacs filled with eggs attached to the 



posterior end of the body. Such egg sacs occur in almost all the 



Copepoda. 



The curious parasitic species are among the most interesting in 

 this order. It is generally the female only that is parasitic, the 

 male being free-swimming and very much smaller; they live on 

 various water animals, but mostly on fishes. These parasites 

 often lose all external resemblance to Crustacea, having worm- 

 like bodies with some of the mouth parts modified into hooks 

 for attachment to their host and for sucking organs. Generally 



