THE ARTHROPODA 



327 



poses, the endopodites are thin-walled plates, and the exopodites and 



the whole first pair of pleopods serve as a gill-cover. 



In the terrestrial Isopoda (Fig. 

 211, B) the wood-lice the gills are 

 adapted for breathing damp air. In 

 these, the first and second gill-covers 

 have air-tubes within them. These 

 function like the tracheae of insects 

 and are therefore physiologically, 

 but not morphologically, compara- 

 ble to tracheae. 



The many different species of 

 Isopoda (except the wood-lice) are 



aquatic. There are many which are parasitic, feeding .on both dead and 



living fish, and fish in turn feed on them. 



A very remarkable finding in the parasite Cymothoidae ( ), 



by Buller, is that the same individual can be developed first as a male 



and then as a female. 



Cryptoniscus ( ) is a more or less shapeless 



sac which attaches itself to the stalk of Sacculina (Fig. 212), and after 



the host (which is itself a parasite) is killed, the new parasite uses the 



"roots" of Sacculina to draw forth its own nourishment. 



The Entoniscidae ( ), parasitic, are usually 



hermaphroditic, although they have small males, called "complemental 



males," attached to themselves. 



Development of the parasitic crustacean, 

 Sacculina carcinus : A, Nauplius stage ; B, 

 cypris stage ; C, adult attached to its host, the 

 crab. Carcinus maenas. (After Hertwig). 



