167 



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 Phreatoicus 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-ventrally 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 PItreatoicus is 

 almost sub-cylindrical and thus resend)les Anthura, Paranthura, 

 and some of the species of /dofea, wliere 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 fe.g. Corophhim, Haplocheira kc.,) and in 

 some such as Icilius and Ipliigenia 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 

 A'uisopoda from the typical Isopoda and the possession of an 

 anterior series of fmir, and a posterior series of tliree is by no 

 means a special Amphipodan character. It is, moreover, probably 

 of little importance from n, systematic point of view, seeing that 

 it is found in such widely difl'erent genera as Phreatoicus^ Stene- 

 triuin. Mi(Aino]>sis, Tanais, and Arcturtts, 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 vei-y short as 

 in most Amphipoda. Tlie uropoda again present no greater 

 resemblance to the Amphipoda than to several of the Isopoda such 

 as Asellus. 



In the possession of an abdomen formed of six distinct and well 

 developed segments Plireatoicus certainly differs from the greater 

 nundjer of the Isopoda, but this character is also possessed by the 

 Apseudidce and the Tanaida', which are usually classed among the 

 Isopoda, and by Lininoria, and also to a greater or less extent by 

 many of the Cymothoidce and Oniscidce &c. On the other hand 



* Eevisioa of the Australian Isopoda, Proe. Linn. Soc, N.S.W., IX.. 

 part iv., p. 10. 



