Tribe 6. 



EPICARIDA. 



The Isopoda belonging to this tribe are all parasitic forms, and their 

 hosts are invariably other Crustacea of different orders ; hence the above name 

 of the tribe. They are the most degradated forms of Isopoda, and in some instances 

 the fully-grown female represents merely a simple sac filled with ova, and 

 without any trace of segmentation or of limbs, so that the idea of its being 

 a crustacean animal would hardly suggest itself, if the development were not 

 known. In all the forms a more or less pronounced regressive metamorphosis 

 has been observed, comprising at least two distinct larval stages, very dis- 

 similar both in their general appearance and in the structure of the 

 several appendages, and equally different from the adult animal. As will 

 be shown further below, there is in all probability in most of the forms even a 

 3rd intermediate larval stage, the Microniscus stage, which, however, has not 

 hitherto been recognized as such, but has been wrongly regarded as an adult form 

 of Epicarida representing even a distinct family. The sexual dimorphism in 

 all the forms is very pronounced. The male in all cases, as compared with 

 the adult female, is of diminutive size, and is generally found clinging to the 

 genital region of the female like a parasite. It is also of an appearance very 

 different from that of the female, retaining, as it does, several of the larval 

 characters, and in some instances it does not differ from the last larval stage 

 except by the presence of generative organs. In certain cases (Entoniscidse), the 

 parasite penetrates to the inner organs of its host, looking merely like an 

 endoparasite, and in all such cases the body of the female is found to be greatly 

 deformed. But when the parasite retains its character as a true ectoparasite, 

 the female also exhibits, as a rule, some more or less pronounced deformity — and 

 is often conspicuously asymmetrical, even when the segmentation of the body is 



