206 DR. C. CHILTON ON THE SUBTEERANEAN 



iscliia would not necessarily prove that the animal is not an Isopool (unless, indeed, we 

 remove the Apseudidte and the Tanaidae to the Amphipoda, and this, notwithstanding 

 Gerstaecker's opinion, does not seem to be desirable), the fact that it possesses long ischia 

 in all the appendages of the perseon is a pretty clear indication that it is not an 

 Amphipod. 



It may also be pointed out that although the first appendage of the perfeon of Phrea- 

 toicns is subchelate, as in the Amphijioda, the second appendage resembles the third in 

 being quite simple, while in the Amphipoda the second appendage is usually subchelate 

 like the first, or, if not actually subchelate, it shows a greater tendency to resemble the 

 first leg than the third. 



(5) In the possession of a long pleon of six separate segments, Phreatoicns certainly 

 resembles the Amphipoda, and differs from most Isopods, but the same character is also 

 possessed by the Apseudida? and the Tanaidae, and by the genus Hysswra [106, p. 128] in 

 the Anthuridpe ; and in many other Isopods, such as Limnoria and many of the Cymo- 

 thoidfe, Oniscidaj, &c., the plcon, though not long, is composed of more or less separate 

 segments. 



The reasons given above will, I think, be quite sufficient to prove that there are no 

 good grounds for classing Plireatoicus with the Amphipoda ; for positive evidence that it 

 is an Isopod it will be sufficient to take the following : — 



(1) The first five pairs of pleopoda are branchial, and there are no branchial plates 



attached to the ap^iendages of the peraeon. The pleopoda themselves are 

 quite different in form from those of the Amphipoda. 



(2) The whole of the mouth-j)arts are distinctly Isopodan in character, and quite 



different from those of the Amphipoda. 



(3) As shown above, the legs are really Isopodan, though at first sight they may 



appear to be Amphipodan. 



(4) The telson is joined to tlie sixth segment of the pleon, as is usually the case with 



the Isopoda, but not with the Amphipoda. It is quite true, as Stebbing [108, 

 p. 549] has pointed out, that this is also the case with certain Amphipoda, the 

 Hyperina for example ; but this is exceptional, and since Plireatoicus is certainly 

 not one of the Syiicrina, it does not affect the present argument. The large 

 size and the form of the telson itself also clearly mark it off from the 

 Amphipoda. 



It will be noticed that, in considering the differences between the Isopoda and Amphi- 

 poda, I have confined myself to external characters. Other important differences in the 

 internal anatomy have l)een pointed out by Blanc [12], but the material at my disposal 

 did not permit of my testing Plireatoicus by these points, even if I had possessed the 

 necessary skill to do so. 



We have now to compare Plireatoicus with the other Isopoda to see what place it 

 should take among them. It will be sufficient if we compare it with the Tanaidae, 

 AnthuridtB, Idotcidae, and the Asellidae. 



It agrees with the Tanaidte in the cylindrical form of the body, in the direction of the 



