1(3 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvii. 



Quite a different mode of respiration is effected in the tribe Chelif era 

 from what is found in the other Isopoda. The pleopoda are used for 

 swimming- and are never branchial in character, the respiratory func- 

 tion being carried on by means of branchial chambers situated under 

 the sides of the posterior part of the carapace (Stebbing). 



The Oniscida^, a terrestrial family, have air sacs developed in the 

 pleopoda, sometimes the opercular branch of two or three pairs being 

 provided with trachea, and sometimes the opercular branch of all the 

 pairs containing trache^B. This adaptation is probably due to their 

 aerial mode of life. 



Certain genera of Chelifera are remarkable for having no pleopoda, 

 as, for example, TanaeUa Norman and Stebbing, StnmgyJura G. O. 

 Sars, and Anarthrura G. O. Sars. This character is usually correlated 

 with a fusion of the abdominal segments. The genera Cryptocope G. O. 

 Sars, and Ilaplocope G. O. Sars have rudimentary pleopoda in the 

 female. There are but three pairs of pleopoda in Tanals Audouinand 

 Milne Edwards and in the genus Parajyse^ides Sars there are but four 

 pairs. The genera PseudotanaisG. O. Sars and LeptognatJiia G. O. 

 Sars have pleopoda which are all developed and ciliated or altogether 

 absent in the female, but always fully developed and ciliated in the 

 male. Pagurapseudes Whitelegge has never more than three pairs of 

 pleopoda, often only one pair, especially in the female. 



The first pleopoda are wanting in both the Tylidte and the Helleriidai 

 and in the females of the Asellidte. Leiop)us Beddard, a genus of the 

 Apseudidse, has one of the branches of all the pleopoda two-jointed, 

 and the genera Phreatoicus Chilton, Phreatoicopsis Spencer and Hall, 

 and Ilypsimetopus Saj^ce have the outer branch of the second and 

 following pairs of pleopoda also two-jointed, this jointed character of 

 the pleopoda not being found elsewhere among the Isopoda, though a 

 feature of the Amphipoda. 



In Bathynoonus Milne Edwards there are supplementary ramified 

 branchiae at the bases of the pleopoda. 



As previously stated, the inner branch of the second pair of pleopoda 

 carries a stylet in the males. In the Ligiidte, the Oniscidfe, the Tri- 

 choniscida^, and the Armadillididie l^oth first and second pairs of ple- 

 poda are sexual in the males, the inner branches of which are modified 

 into sexual organs, those of the first pair often being coalesced in the 

 Oniscidfe; in the females these branches are rudimentary. 



In the Asellidte the pleon in both sexes has the first pair of pleo- 

 pods quite small, while the outer lamellte of the second pair are very 

 large, forming a sort of operculum, the lamellae of which are not fused 

 together in either sex. The female has four pairs of pleopoda; the 

 male has five pairs, with an additional pair of very small biramous 

 appendages immediately behind the first pleopoda. 



In the Janiridte, the Desmosomidse, and the Munnopsid* the first 

 pair of pleopoda in the female forms a subcircular operculum, while 



