168 
although the segments of the pleon are generally separate and 
well developed in the Amphipoda, the latter never have all the 
lirst six pairs of pleopoda similar to one another and all branchial 
in character as in Phreatoicus , and this fact is of itself, in the 
absence of characters to the contrary, sufficient to show that 
Phreatoicus must be placed among the lsopoda 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 lsopoda, we have next 
to consider its affinities to other lsopoda. When originally 
describing the genus ! briefly compared it with the Tanaidee , the 
Anthuridee , and the 1 dot eider, but did not at that time notice its 
affinities to the AseUiehe , although these arc?, as I hope to be able 
to show, greater than those with any of the other three groups. 
The resemblance to the Tanaidee 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 Tanaidee 
and Phreatoicus 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 Anthuridee is somewhat closer. In 
addition to the general resemblance in the form of the body there 
is a fairly close resemblance in the legs, and the abdomen of the 
Anthuridce though by no means so well developed as in Phreatoicus 
is sometimes composed of more or less distinct segments bearing 
pleopoda not very dissimilar. The mouth parts of the Anthuridce 
are however very different, though in some cases they are no doubt 
specially modified to form an apparatus probably suctorial in 
function, and there are other differences quite sufficient to form 
a pretty wide gap between the Anthuruhe and Phreatoicus. 
With the Idoteidm , Phreatoicus agrees fairly well in the general 
shape of the body, in the antenna* and to a less extent in the 
mouth parts, except that the mandible has a well developed palp 
in Phreatoicus , but none in the species of the Idoteidm. It is 
probable however that this should not be considerod a very im- 
portant point, as in the Amphipoda we have the mandibular palp 
present in widely separated genera while it is sometimes absent 
in others which are otherwise closely similar to genera in which 
the palp is present. Thus the old genus Montagna , Spence Bate, 
is considered by Stebbingto be divisible into Sienothoc , the species 
of which have no mandibular palp, and Met op a in which the palp 
is present.* 
* Report on the “ Challenger ’’ Amphipoda, p. 293. 
