206 DR. C. CHILTON ON THE SUBTEEEANEAN 



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

 remove the Apseudidse and the Tanaida? to the Amphipoda, and this, notwithstanding 

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

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

 Aniphipod. 



It may also he pointed out that although the first appendage of the perseon of Fhrea- 

 toicus is subchelate, as in the Amphipoda, 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, Phreatoicus certainly 

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

 possessed by the Apseudidse and the Tanaidse, and by the genus Hyssiira [106, p. 128] in 

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

 thoida?, Oniscida?, &c., the pleon, though not long, is composed of more or less separate 

 segments. 



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

 good grounds for classing Phreatoicus with the Amjohipoda ; for positive evidence that it 

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



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



attached to the appendages of the perseon. The pleopoda themselves are 

 quite different in form from those of the Amphipoda. 



(2) The whole of the mouth-parts are distinctly Isopodau 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 Amphi^^odan. 



(4) The telson is joined to the sixth segment of the j^leon, as is usually the case with 



the Isopoda, but not ■\rith the Amphipoda. It is quite true, as Stebbing [108, 

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

 Syperina for example ; but this is exceptional, and since Phreatoicus is certainly 

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

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

 Amphij)oda. 



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 been pointed out by Blanc [12], but the material at my disposal 

 did not permit of my testing Phreatoicus by these points, even if I had jiossessed the 

 necessary skill to do so. 



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

 should take among them. It will be sufficient if we compare it with tlie Tanaidse, 

 Anthuridie, Idoteidse, and the Asellidae. 



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



