GENERAL MORPHOLOGY 



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following the moult at which the brood plates are replaced by the fully formed brood 

 lamellae and the eggs are laid, the sterna of the thoracic somites remain soft, their usual 

 function having been taken on by the chitinous middle portions of the brood lamellae. 

 As a result of this, the cavity of the marsupium can be enlarged, not only by the bending 

 downwards of the brood lamellae but also by the pushing upwards of the membranous 

 ventral body wall. In this way accommodation for the increasing size of the developing 

 embryos is ensured. The second part of Beddard's statement that "the young appear to 

 be actually contained within the body of the mother " is inaccurate : the embryos undergo 

 their development within the marsupium and are separated from the body cavity of the 

 mother by the membranous ventral body wall. 



An interesting result of the analysis of the adult female specimens of the species 

 represented in the present collection is that breeding goes on all the year round, and 

 that the number of females in the non-breeding condition is comparatively small. 



The following table gives the details for S. schythei and S. exigtia throughout the 

 year. In the latter species, which is of a small size, the examination of the numerous 

 specimens for the presence or absence of the brood plates was very tedious and was 

 therefore omitted. 



From the above table it may be seen that in every month, when the collections in- 

 cluded these species, females in the breeding phase were present, thus proving that they 

 exhibit no definite breeding season. In this they agree with the Amphipod Gammarus 

 chevreuxi (Sexton, 1924) and differ from the fresh-water Isopod Aselhis aquaticus which 

 appears to have a definite breeding season. 



In Aselhis aquaticus, and also in the members of the fresh-water family Phreatoicidae, 

 females with the narrow plate-like type of brood lamellae as well as those with the broad 

 type are found, and Unwin's observations (1920, p. 335) on Aselhis show that the adult 

 female, during the breeding season, passes through alternating phases of breeding and 

 non-breeding, and that in the latter phase the broad lamellae are replaced by the narrow 

 plate-like type. On the other hand, Gammarus chevreuxi shows no such alternation ; 



