PARASITIC CRUSTACEA 121 



The larval form of the gnathiids is quite a normal looking little 

 isopod, but its mouthparts are modified for piercing and sucking 

 the blood of fish. One such larva, Praniza milloti, has been found 

 attached to the coelacanth, Latimeria. After the larva has gorged 

 itself the third, fourth and fifth thoracic segments are distended to 

 about twice the diameter of the rest of the body. The larvae 

 probably remain parasitic for several months, after which they 

 leave their hosts and take up residence in adult quarters. The adult 

 male has a very large head and mandibles, but the other mouth- 

 parts are reduced and do not function. Neither male nor female 

 feed in the adult state. The females are less modified than the 



Fig. 53. Head of a larva of Para gnat hia formica (Isopoda, 



Family Gnathiidae). Only the bases of the antennae are 



shown. m = mandible. mx = maxilla. 



males; they brood their eggs in internal sacs, and die when their 

 larvae have been released. 



The Gnathiidae are quite a separate group from the Epicaridea, 

 and they do not help in trying to trace the evolution of the latter 

 group. More help is given by members of the family Cymothoidae. 

 These are also parasitic on fish, but only as adults. In some cymo- 

 thoids the females are somewhat asymmetrical when mature, 

 recalling the asymmetry found in the Bopyridae. Another interest- 

 ing feature is that the males often develop into females later in life, 

 showing that the sex is not very strongly determined, and indicat- 

 ing how the lability of the sex in the Bopyrids may have arisen. 



