CRUSTACEA OF NEW ZEALAND. 207 



leo-s, and in the possession of a pleon formed of six separate secjments. All these 

 characters are, however, separately shared hy other groups, and the differences in other 

 respects are very considerable, and we may safely conclude that Phreato'icus has no very 

 close affinity with the Tanaidae. 



The Apseudidro, which rank close to the Tanaidre, do not seem to present any greater 

 affinity to Phreatoictis. 



The resemblance of the Anthuridfe is, however, somewhnt greater. There is a fairly 

 good general resemblance in the shape of the body and in the legs, and though the pleon 

 is usually short in the Anthurida?, it is often composed of separate segments, and these 

 may be of fair length, as in the genus Hyssura, Norman and Stebbiug [lOG, p. 128]. 

 The mouth-parts are very different, being specially modified in the Anthuridfe for the 

 purpose of suction, and this, combined with differences in the pleopoda, uropoda, &c., is 

 sufficient to make a pretty wide difference between the two. 



With tlie Idoteida3, Phreatoicus agrees in the shape of tlie body, in the antennae, and 

 to some extent in the mouth-parts. In the Idoteida? these are more modified tlian in 

 Phreatoicus, though formed on the same plan, and the mandible has no palp. It is 

 probable, however, tliat the presence or absence of a mandibular palp is not a point of 

 great systematic importance, for in the Amphipoda we have genera, in other respects 

 closely similar, differing in this point ; thus tlie old genus Ilontagua, Spence Bate, has 

 been divided into Stenothoe, in which the mandible has no palp, and 3Ietopa, in which 

 the palp is present [108, p. 293]. A much more important difference is found in the 

 structure of the pleon and the uropoda. In the Idotcidtc the segments of the pleon, 

 except the last, are usually very short and more or less coalesced and the uropoda form 

 flat plates covering up the pleopoda. There are, however, sufficient signs that the pleon 

 of IdoteidtB has been derived from a pleon formed of separate segments, and that the 

 uropoda, though now very different, are simply a modified form of the typical uropoda 

 consisting of a peduncle and two rami ; and it is quite probable that the special modifications 

 of the Idoteidse in these respects are of comparative recent date, and that their ancestors 

 presented a much closer resemblance to Phreato'icus tlian the present Idoteidae do. 



The Arcturidae, again, might be compared with Phreatoicus in much the same way, 

 but they present a further resemblance in the legs, which, though very different in 

 form, are very distinctly divided into an anterior series of four and a posterior series 

 of three. 



When we come to compare Phreatoicus with the Asellidre we at once see a very "-reat 

 difference in the form of the body, but on closer examination the resemblances are seen 

 to be much more numerous and much closer than might at first siglit be expected. The 

 head, antennce, mouth-parts, and the legs are all in pretty close agreement; the resem- 

 blance in the mouth-parts is indeed somewhat striking, and altliough the legs of the last 

 three pairs are more Amphipodan and flattened, there is a general resemblance in the 

 relative lengths of the different joints to those of Asellus. In describing P/«-f«/oit';/s 

 australls I took Sars's description of Asellus aquaticus [91, pp. 96-100] as my guide, and 

 was able to follow it pretty closely. The uropoda, again, are not very unlike those of 

 Asellus, and the pleopoda of Phreatoicus appear to present more resemblances to those 



