1G7 
unlike that common among the Amphipoda. (5) The pleon is 
formed of six: separate segments and is better developed than 
most Isopods, 
I think these are all the points in which Phreatoicns specially 
resembles the Amphipoda and an examination of them shews that 
none is of any particular importance in its bearing on the systematic 
position of the genus. Most of the Isopoda are, it is true, more 
or less dorso- ventral ly compressed, and I do not now recollect any 
one in which there is any lateral compression as in Phreatoicus , 
but here the lateral compression is not great and is chiefly confined 
to the pleon where the downward prolongation of the pleura 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 pereion of Phreatoicus is 
almost sub-cylindrical and thus resembles Anthura , Paranthnra , 
and some of the species of Mote, a, where there is no dorso- ventral 
compression. On the other hand there are genera among the 
Amphipoda in which the body is more or less cylindrical and shows 
no lateral compression (e.g. Corophium, Haplocheira &c.,) and in 
some such as Icilius and Iphigenia the body is very much flattened 
as in the Isopoda. 
The division of the appendages of the pereion into an anterior 
and a posterior series has been used by Dana in separating the 
Anisopoda from the typical Isopoda and the possession of an 
anterior series of four , and a posterior series of three is by no 
means a special Amphipodan character. It is, moreover, probably 
of little importance from a systematic point of view, seeing that 
it is found in such widely different genera as Phreatoicus , Stem- 
trium , Munnopsis , Tanais , and A returns, and its adoption as the 
chief bond of connection between a number of forms results, as 
Mr. Haswell has pointed out, in “an extremely artificial arrange- 
ment.'’* In connection with the fourth point, the general Amphi- 
podan appearance of the legs and the uropoda, I have already 
shown that this is more apparent than real, as the legs all have 
the ischios well developed and fairly long instead of very short as 
in most Amphipoda. The uropoda again present no greater 
resemblance to the Amphipoda than to several of the Isopoda such 
as Aseltus. 
In the possession of an abdomen formed of six distinct and well 
developed segments Phreatoicns certainly differs from the greater 
number of the Isopoda, but this character is also possessed by the 
Apseudidce and the Tanaidce , which are usually classed among the 
Isopoda, and by Lirnnoria, and also to a greater or less extent by 
many of the Cyuiothoidce and Oniscidce &c. On the other hand 
* Eevision of the Australian Isopoda, Proc. Linn. Soe., N,S,W„ IX., 
part iv., p. 10. 
