thus meriting the name of true gnathopoda, but in some instances the 2 suc- 

 ceeding pairs assume a similar prehensile nature (^Egidse, Cirolanidse, Idotheidse). 

 In the group Chelifera, the 1st pair of legs attain their greatest development, 

 exhibiting the character of true chelipeds, as in the higher Crustacea. The number 

 of legs is 7 pairs, as in the Amphipoda, with the one exception of the family 

 Gnathiidce, in which only 6 pairs are counted, the last pair being wholly absent. 

 This, as is well known, is also the case with the young of all Isopoda, immedi- 

 ately after being hatched. The coxal plates to wbich the legs are appended, are 

 far from being so distinct in the Isopoda as in the Amphipoda. In a great 

 number of forms not even a trace of these plates is to be seen, and, when 

 present, they are always so firmly connected with the corresponding segments, 

 as scarcely to admit of being detached from them by dissection., In no case 

 are branchial lamellae found to be appended to these plates. The appendages 

 of the metasome are, as a rule, 6 pairs, as in the Amphipoda. Of these 

 the 5 anterior ones are in some instances natatory in character. But in the 

 greater part of the Isopoda their function has changed to be more or less ex- 

 clusively respiratory, and in such cases they lie densely crowded beneath the shield- 

 like terminal portion of the metasome. In the group Asellota the 1st pair of 

 these appendages are, as a rule, peculiarly modified, being in the female coalesced 

 to a single thin opercular plate, covering the succeeding jjairs, whereas in the 

 male they are transformed into complicated copulative organs. Only the last pair 

 can properly be termed uropocla. Their structure is rather variable in different 

 Isopoda, affording excellent systematic characters. The telson is distinctly defined 

 only in the Anthuridce. In all other Isopoda it is fused with the preceding seg- 

 ment, and in some cases all the segments of the metasome are coalesced to a 

 single shield-like plate, the so-called urns. 



The systematic arrangement adopted in the present work is that pro- 

 nosed by the present author in 1882 (Oversigt af Norges Crustaceer I). I give 

 below a Synopsis of the 6 Tribes into which the Isopoda, according to this ar- 

 rangement, are divided, each tribe being defined by 3 characters, viz., those of 

 the 1st pair of legs, the uropoda, and the pleopoda: 



