THE CRUSTACEA 



ecdysis takes place, the oostegites are cast off and receptacula are 

 again developed, the animal reverting to the condition in which 

 it was before impregnation. 



The arrangement of the oostegites which form the marsupial 

 ch'amber in all normal Isopoda has already been described. In 

 certain cases, however, brood-pouches are formed in other ways. 

 In the section Cryptoniscina among the Epicaridea a series can be 

 traced in which the oostegites diminish in size and finally disappear, 

 their place being taken by lateral folds of the body. The term of 

 the series is given by Hemioniscus, in which the brood-cavity is 

 from the first completely closed, arising by delamination in a 

 thickening of the ventral ectoderm. 



A remarkably varied series of adaptations for carrying the 

 eggs and young have recently been made known in the family 

 Sphaeromidae. In some members of this family the marsupium is 

 formed by the oostegites in the usual manner, but in others special 

 brood-pouches are formed by invaginations of the ventral integument, 

 and in some cases here also oostegites are quite wanting. 



In addition to their protective function in sheltering the eggs 

 and young, it has been suggested that the oostegites may in some 

 cases supply nourishment to the developing embryos. In certain 

 Oniscoidea papilliform projections from the sternal surface of the 

 thoracic somites have also been credited with this function. 



The testes, in the majority of Isopoda, consist each of three 

 follicles (Fig. 128, ts) opening into a common vas deferens (v.d). 

 Only in a few cases is the number of follicles reduced to one on 

 each side. The external openings are generally set on papilliform 

 or tubular processes (penes) (Fig. 125, p), which may be fused into 

 one (Oniscoidea, except Ligiidae, Arcturidae). In the Epicaridea 

 the penes are commonly absent and the aperture may be paired or 

 single, but in Priapion a bifurcated penis of great size is present. 



The position of the penes sometimes departs a little from the 

 general rule among Malacostraca in so far as they may spring, not 

 from the last thoracic sternum, but from the articular membrane 

 between it and the first abdominal somite, and may even be 

 attached to the sternum of the latter. It is very improbable, 

 however, that the vasa deferentia ever perforate the copulatory 

 appendages of the second pleopods as they have been stated to do 

 in the Tylidae (Oniscoidea). 



The occurrence of protandrous hermaphroditism has been 

 demonstrated in certain genera of the sub -family Cymothoinae 

 among the Cymothoidae, and of the tribe Cryptoniscina among 

 the Epicaridea. It is not known to occur in the other sub-families 

 of the Cymothoidae ; and though its limits within the group 

 Epicaridea are not exactly known, it is certain that many of the 

 families, probably the whole of the tribe Bopyrina, are definitely 



