CEUSTACEA OF NEW ZEALAND. 205 



I think these are all the points in which Phreatoicus specially resembles the Amphi- 

 poda, and an examination of them shows that none is of any particular importance in its 

 bearing on the systematic position of the genus. I will take the points one by one 

 under their appropriate numbers as given above. 



(1) Most of the Isopoda arc, it is true, more or less dorso-ventrally compressed, and I 

 do not know of any one in which there is any lateral compression as in FJireatuiciis ; but 

 here tlie latei'al compression is not great, and is chiefly confined to the plcon, where the 

 downward prolongation of the pleux-a is no doubt a special adaptation for the protection 

 of the pleopoda, and may very well have arisen quite independently of the similar 

 adaptation in the Amphipoda. The perajon of Phreatoicus is subcylindrical, and thus 

 resembles Aufhura and Paranthura, and other genera of the Anthuridte [lOG], and some 

 species of Idotea, such as Idotea elongata [21, p. 198], in which there is no dorso- ventral 

 compression. On the other hand, lateral compression is by no means universal among 

 the Amphipoda ; there are many genera where the body is more or less cylindrical, as in 

 Caprella &c., Corophium, Haplocheira, and many others, while there are also some, 

 such as Icilius, Iphigenia, and Cyamus, in Avhich the body is much flattened, as in most 

 Isojjoda. 



(2) This point has practically already been disposed of in the consideration of (1), and 

 I need only add that Phreatoicus has the pleura of the first jive segments of the pleon 

 produced downwards, while in the Amphipoda it is only in the first three segments that 

 the pleura are so produced. 



(3) The division of the appendages of the peraeon into an anterior series of four and 

 a posterior series of three has been used by Dana in separating the Anisopoda from the 

 typical Isopoda, and it is by no means a special Amphipodau character. It is, more- 

 over, probably of little importance from a systematic jioint of view, seeing that it is 

 found in such widely different genera as Phreatoicus, Stenetrium, Munnopsis, Tanais, 

 and Arcturus, and its adoption as the chief bond of connection between a number of 

 forms results, as Professor Haswell has pointed out, in " an extremely artificial arrange- 

 ment " [55, p. 10]. 



(•i) The appendages of the peneon appear at first sight undoubtedly Amphi])odan, but 

 here, again, a closer examination shows that the resemblance is merely superticial, for in 

 all the legs we find that the ischium is fairly long, often as long or even longer than the 

 preceding joint, the basos, while in almost all the Amphipoda the ischium is quite short, 

 often transverse. In the possession of moderately long iscliia, Phreatoicus agrees with 

 most other Isopoda. I am not aware that anyone but myself has drawn attention to this 

 difference between the Isopoda and the Amphipoda ; but it appears to be one of very 

 general application, though, of course, there are some exceptions to it as to every other 

 rule in Natural Science. Thus, in the Apseudidae and the Tanaidie [106], the ischium is 

 usually short, while in a few cases in the Amphipoda it is long, as in the second gnatho- 

 poda of the Lysianassidse, and also in the second guathopoda of Seba [108, j). 783], and 

 perhaps in a few others. But in all these cases that I know of in the Amphipoda the 

 long ischium is found in one pair of legs only, and I know of no Amphipod that lias the 

 ischium in each pair of legs long as in the Isopoda ; so that, while the possession of short 



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