DEaENEKATION OF ORGANS. 



81 



only by the anus ; but when the young larvEe of the Crustaceans 

 have once found their way in— which is not difficult by reason of 

 the strong and rhythmical indraught of water through the cloaca 

 — they never seem to quit the situation of their own choice ; at 

 the same time they greatly irritate the organ, and as they grow 

 they stop up the tubular vessel more and more till at last 

 serious degeneration of the organ is induced (fig. 21, 6). The 

 main trunk is greatly distended, while the lateral branches, 

 which usually form a highly ramified structure, dwindle alto- 

 gether, and are visible only as thin filaments, sometimes feebly 

 branched. The young larvsB now produced are excluded, and 

 become wandering bodies, in obedience to the law which 



Fia. 22.— Zoea stage of the larva of Pinnotheres HolothuMw. 



governs all Ento-parasite.s ; this they do under the form 

 of the larva, or Zoea (fig. 22), which is common to all crabs, 

 and they have the well-developed eyes of the typical character. 

 Even when they enter the animal, they still preserve these eyes ; 

 but as they grow they gradually iDecome blind or half-blind, the 

 brow grows forward over the eyes, and finally covers them so 

 completely that, in the oldest individuals, not the slightest 

 trace of them, or of the pigment, is to be seen through the 

 thick skin ; while at the same time the eyes seem to undergo a 

 more or less extensive retrogressive metamorphosis. 



' The instances here adduced show very clearly that the 

 absence of light sometimes occasions degeneration from disuse, 



