AMPHIPODA. 453 



lobed maxillae also have as a rule a short, two-jointed palp, while the 

 maxillae of the second pair are reduced to two lamellae of considerable 

 size attached to a common base. The maxillipeds fuse to form a sort 

 of underlip, which is either tri-lobed (Hyperina} or bears upon a com- 

 mon basal portion an internal and external pair of lamellae, of which 

 the latter may be considered as the basal joint of a large multiar- 

 ticulate and frequently pediform palp (Crevettina and Lcemodipoda). 



Delicate lamellae or tubes, which are attached to the coxal joints of 

 the thoracic legs, function as gills ; the active movements of the 

 abdominal swimming feet cause a constant renewal of the water 

 around them. In the female there are in addition to the gills 

 lamellar plates (oostegites), which are applied together under the 

 thorax to form a brood-pouch. 



The males are distinguished from the females not only by the 

 absence of the oostegites, but chiefly by the stronger development of 

 the prehensile hooks on the anterior thoracic feet and the different 

 formation of the antennae. 



The eggs pass into the brood-pouch and there develop. The yolk 

 sometimes (G. locusta and other marine species) undergoes a com- 

 plete segmentation. Sometimes (G. pulex), after a superficial seg- 

 mentation, a peripheral cell-layer is separated, which develops into 

 a delicate blastoderm beneath the egg membrane. A ventral 

 primitive streak is then formed, and on the dorsal side, beneath a 

 differentiation which has been erroneously taken for a micropyle, 

 a peculiar globular organ makes its appearance ; this is the first rudi- 

 ment of the cervical gland (dorsal organ), which is confined to em- 

 bryonic life. The appendages are developed from before backwards 

 on the ventrally flexed body of the embryo. The young animals 

 usually possess at hatching all their appendages and in all essential 

 points have the structure of the adult animal, but the number of 

 joints of the antennae and the special form of the legs still present 

 differences. In the Hyperina alone the just hatched young may be 

 without abdominal feet and differ so much in their form from the 

 adult that they may be said to undergo a metamorphosis. 



The Amphipoda for the most part live in fresh and salt water 

 and lead an independent life (the presence of Arctic species in the 

 Swedish and Norwegian seas is very interesting). Some, however, 

 live in tubes (Cerapus), others in holes gnawed in wood (Chelura). 

 The large size of the deep-sea forms is of special interest ; amongst 

 these a Gammarid, allied to the genus Ipkimedia, and Cystosoma 

 Neptuni (Hyperidce) become several inches in length. The Hyperina 



