No. 26.] ARTHROSTRACA OF CONNECTICUT. 3 1 



third and fourth segments. They are inserted on the inner 

 surface of the coxal plates except in the CaprelHdea and 

 Phronimidse where the attachrrient is directly to the ventral wall 

 of the thorax. The insertion is nearer the middle line than the 

 gills, so that when the brood pouch is filled with eggs the gills 

 remain free to oscillate and receive fresh water. 



In form these plates are rather long, tongue-shaped plates the 

 margins of which are always provided with numerous long, slen- 

 der prolongations like exceedingly flexible setae. These prolonga- 

 tions intertwine with each other so that the lamellae form a 

 kind of basket which receives the eggs from the oviducts. 



SEXUAL DIFFERENCES. 



The sexes of the Amphipoda are always separate. Usually 

 they are readily distinguished by the large marsupial plates, or 

 oostegites, of the females which support and protect the eggs 

 and immature young. Besides this difference in the two sexes, 

 the males are usually larger than the females and are provided 

 with more sensory spines and setas. It is in the form of the 

 second antennae and gnathopods, however, that the secondary 

 sexual characters are most conspicuous. In striking contrast to 

 the general rule that the second antennae of the males bear more 

 setas than those of the females is the case of Corophimn, in which 

 the second antennae of the male have none on the peduncle while 

 those of the female have many. On the first antennas of Am- 

 pelisca and the Lysianassidae, calceoli are present in the males 

 but not in the females. 



The first gnathopods of Orchestia, Cerapus, and Microdeuto- 

 pus are of vastly dififerent form in the male and female, as may be 

 seen by reference to the descriptions of the forms in question. 

 In other genera, like Amphithoe, Hyale, and Gammarus, for 

 example, the sexual differences in the first gnathopods are in- 

 significant. 



The second gnathopods are strikingly different in the two 

 sexes in Talorchestia, Orchestia, Metopa, Elasmopus, Amphithoe 

 longimana, Jassa, Ischyrocerus, and Ericthonius. 



In Chelura terebrans, the second and third uropods are quite 

 distinct in form in the two sexes; the second pair is provided 

 with long setae in the male which are lacking in the female; the 



