170 



some species have the abdomen composed of four or live separate 

 segments, other species with only two or three segments, and 

 others again, like Idoiea elongata, with the abdomen formed of 

 a single piece.* The development of an operculum from the 

 first pair of pleopoda would natuially follow from the flattening 

 of the abdomen which would leave the pleopoda much exposed 

 below. Thus riireatoicus appears to have preserved the fully 

 developed abdomen which must have been possessed by the 

 ancestors of the Asellida', while in the latter this has been 

 specially modified in accoi'dance with the genei'al flattening of the 

 body, which would render a long jointed abdomen unsuitable and 

 a source of danger to the animal, especially by the exposure to 

 which it would subject the pleopoda. 



There is one genus — Lunnoria — sometimes classed under the 

 AseUidif, which difl'ers from them and resembles Plireatoicus in 

 possessing an abdomen of six separate segments, the pleopoda also 

 are unprotected. Limnoria however resembles the other J se/ZifA^- 

 in the flat depressed body, and the segments of the abdomen though 

 separate are short, so that Limnoria may very well be looked 

 upon as an intermediate link between FJireatoicus and the A sellidce. 

 The great diflerence between the two latter is caused by tiie fact 

 that the body in the Asel/idce is flat and depressed, while it is 

 somewhat compressed in Plireatoicus, and that consequently the 

 AsellidfK are always represented as seen from above, while 

 Phreatoicus is usually seen from the side. This diflerence in the 

 form of the body though it gives quite a diflerent appearance to 

 the animal is probably not of very much importance fi'om a 

 systematic point of view, thus some of the species of Idotea differ 

 very much in the shape of tlie body, and I think we must place 

 Plireatoicus near to the Ase/lida; but forming a new family, the 

 Phreafoicidir, which bears to the AseUido' much the same relation 

 that the CapreUida' do to the Ciianiido' among the Amphipoda. 

 Lhnnoria will perhaps best l)e placed as a special subdivision of 

 the.4se/^i<;i?^'', connecting them to some extent with the Phreatoicida'. 



I have not compared Plireatoicus with some othei- families of 

 the Isopoda that it might well be compared witli, such as, for 

 intance, the ^Kfjida- and tlie Spliaeroinida', but from what has ))een 

 already said it will readily be seen that it possesses various 

 characters in counnon with these as well as with those already 

 considered inasmuch as it preserves to a large extent the typical 

 characters of the Tsopoda, and tlms occupies a more or less central 

 position, around which the other families may be grouped. 



The relation of Plireatoicus to its nearest allies may be graphi- 

 cally represented by the following hypothetical genealogical tree: 



* Sue " Kevision of the Now Zealand Iduteidie," Trausautions N. Z. Inst., 

 XXII., p. 199. 



