204 DR. C. CHILTO^' ON THE SUBTEEEANEAN 



Affinities of Plireatoicus. 



Phreatoicus presents so many peculiarities that it is difficult to determine its exact 

 systematic position, and its affinities must therefore be discussed at some length. The 

 followino" account is partly reproduced from my paper on Phreatoicus australis [26], but 

 it has been revised and to some extent made more complete. 



When I originally described the genus Phreatoicus in 1882 [23], I placed it in the 

 Isopoda, and pointed out various separate resemblances to the Idoteidse, the Anthuridse, 

 and the Tanaidse, and also drew attention to the several resemblances to the Amphipoda ; 

 but after doing this I left the exact position of the genus among the other Isopoda 

 an open question for the time. When preparing the " Critical List of the Crustacea 

 Malacostraca of New Zealand" [111, p. 151] Mr. Thomson, judging from the general 

 appearance (he had not had an ojiportunity of examining specimens), was inclined to 

 place it under the Amphipoda, and, as I did not agree with this opinion, it was arranged 

 that the genus should be placed between the Amphipoda and the Isopoda under a 

 separate heading with the following note: — "The systematic position of this singular 

 Crustacean is doubtful. In general appearance I was inclined to place it among the 

 Amphipoda, but from the fact of the first five pairs of pleopoda acting as branchial 

 organs, and from the absence of any such organs attached to the pereion, Mr. Chilton 

 places it among the Isopoda. — G. M. T." [Ill, p. 151]. 



Unfortunately, however, the separate heading was omitted by some error, probably on 

 the part of the printer, and the genus therefore appears under the last family of the 

 Amphipoda, viz. the PlatyscelidjE, as though it belonged to that family. It is no wonder, 

 therefore, that the Rev. T. R. R. Stebbing, in his notice of the " Critical List," says, in 

 speaking of Phreatoicus, " I do not know what are the special reasons for classing it 

 among the Platyscelidse." He also says, "The list [i. e. our ' Critical List '] continues 

 with ' Suborder II. Isopoda. Tribe I. Auisopoda. Pam. I. Tanaidte ; ' and probably 

 the affinities of Phreatoicus will eventually prove to be rather with the TanaidoB than 

 with the Hyperina" [108, p. 587]. In another reference to the species Phreatoicus 

 tijpicus, Mr. Stebbing calls it " a singular well-shrimp of a new genus and species, which 

 appears to be an Isopod with some remarkable Amphipodan affinities " [108, p. 543]. 



At first sight Phreatoicus certainly does look very like an Amphipod, but on exami- 

 nation this is found to be due to superficial resemblances only, and not to any real 

 affinity to that group. These resemblances appear to be as follows : — 



(1) The body, especially in the pleon, is more or less laterally compressed. 



(2) The pleura of the segments of the pleon are produced do-miwards, so as to 



protect the pleopoda on either side, just as in the Amphipoda. 



(3) The legs of the perseon consist of an anterior series of four and a posterior series 



of three. 



(4) The general appearance of the legs and of the uropoda is not unlike that 



common among the Amphipoda. 



(5) The pleon is formed of six separate segments, and is better developed than in 



most Isopoda. 



