328 



General Biology 



Fig. 212. 

 Development of the parasitic crustacean, 

 Sacculina carcmus : A, Nauplius stage; B, 

 :ypris stage; C, adult attached to its host, the 

 :rab, Carcinus maenas. (After Hertwig.) 



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 ( ), which are parasitic, are 



usually hermaphrodite, although there are small males, called "com- 

 plemental males," attached to the larger female. 



