168 



although the segments of the pleon are generally separate and 

 well developed in the Amphipoda, the latter never have all the 

 tirst six pairs of pleopoda similar to one another and all branchial 

 in character as in Plirratoicus, and this fact is of itself, in the 

 absence of characters to the contrary, sufficient to show that 

 Plireatoicus must be placed among the Isopoda and not under the 

 Amphipoda. 



Having now discussed the supposed Amphipodan affinities of 

 Phreatoicus, and having come to the conclusion that they are by 

 no means sufficient to remove it from the IsojDoda, we have next 

 to consider its affinities to other Isopoda. When originally 

 describing the genus I briefly compared it with the Taiiaid(p, the 

 AiithiLr'id(P, and the Idoteichf, but did not at that time notice its 

 affinities to the Aselluhr, although these are, as I hope to be able 

 to show, greater than those with any of the other three groups. 



The resemblance to the Tanaidce is not great, and is confined 

 to the more or less cylindrical form of the body, the direction of 

 the legs and the possession of an abdomen of six distinct and well 

 developed segments. All of these characters are however separately 

 shared by other groups, and the differences between the Tanaidce 

 and l^lireatoious in other respects (which it is unnecessary to point 

 out) are very great, so that we may safely conclude that there is 

 no very close affinity between the two. 



The resemblance to the AntJnirida' is somewhat closer. In 

 addition to the general resemblance in tlie form of the body there 

 is a fairly close resemblance in the legs, and the abdomen of the 

 AiitJiuridd' though by no means so well developed as in PhrPMtoicus 

 is sometimes composed of more or less distinct segments bearing 

 pleopoda not very dissimilar. The mouth parts of the Autliuridxf 

 are however very different, though in some cases they are no doubt 

 specially modified to form an apparatus pro))ably suctorial in 

 function, and there are other diffei'ences quite sufficient to form 

 a pretty wide gap between the Antliuridcf and Plireatoicus. 



With the Idoteid(f', Phreatoicus agrees fairly well in the general 

 shape of the body, in the antenna? and to a less extent in the 

 inouth parts, except that the mandible has a well developed palp 

 in Plireatoicus, but none in the species of the Idoteidie. It is 

 proljable however that this should not be considered a very im- 

 portant point, as in the Amphipoda we have the mandibular palp 

 present in widely sejjarated genei'a while it is sometimes absent 

 in others which are otherwise closely similar to genera in which 

 the palp is pi-esent. Thus the old genus Mo')U(((jaa, Spence Bate, 

 is considered by Stebbingto be divisible into Stenothot, the species 

 of which have no mandiljular palp, and Mi'fopa in which the palp 

 is present.* 



* Report on the " Challeno^er '' Amphipoda, p. 293. 



