238 THE CRUSTACEA 



of the ectoderm, the cells of which become pyriform, projecting 

 inwards and connected with the exterior by a narrow neck. The 

 larval cuticle, which is formed soon after the differentiation of 

 appendages has begun, remains adherent to the dorsal organ after 

 separating from the rest of the ectoderm. Later, the cells lose 

 their apparently glandular character and become invaginated as a 

 thin-walled vesicle, which persists after the embryo is freed from 

 the egg-membrane. 



The dorsal curvature of the blastoderm as it lies upon the 

 spherical surface of the yolk is early exchanged for a ventral 

 curvature as the abdominal region of the developing embryo 

 becomes folded downwards and forwards. The embryo is not 

 liberated from the egg-membrane until the body and appendages 

 have assumed more or less their final form. The young, however, 

 remain within the marsupium for some time longer, leaving it 

 finally only at the ecdysis which precedes the next act of oviposi- 

 tion. The accounts of some older writers, according to which the 

 young after leaving the marsupium of the parent returned to it for 

 shelter when alarmed, have not been confirmed by any modern 

 observer. 



The post-embryonic development in most Amphipoda consists 

 mainly in the gradual assumption of secondary sexual characters 

 and other features of subordinate importance. Only in the 

 Hyperiidea, and notably in the Platyscelidae, do the changes of 

 form occurring after the young leave the brood-pouch deserve the 

 name of metamorphosis. 



REMARKS ON HABITS, ETC. 



The Hyperiidea and Caprellidea are exclusively marine (the 

 reported occurrence of a Caprellid in the Lake of Geneva rests 

 on insufficient evidence), as are also the great majority of the 

 Gammaridea. The Hyperiidea are pelagic in habitat, sometimes 

 ranging from the surface to great depths, and having often an 

 exceedingly wide horizontal distribution. The Caprellidea, though 

 for the most part inhabiting shallow water and almost or quite 

 without the power of swimming, include some species of almost 

 cosmopolitan range. The Ingolfiellidea have also a very wide 

 distribution, for the two species which at present compose the 

 sub-order come from Davis Straits and from the Gulf of Siam, at 

 depths of 1870 fathoms and of 1 fathom respectively. 



The marine Gammaridea are rarely pelagic ; they are abundant 

 in the littoral region and penetrate to great depths. The vast 

 abundance of individuals and species in Arctic and Antarctic seas 

 is especially noteworthy. As regards the freshwater species, the 

 predominance of the genus Gammarus and its nearest allies must 



