202 DK. C. CHILTON ON THE SUBTEERAN^EAN 



(2) The telson proper is much shorter and rounder. 



(3) The inferior margin of the sixth segment of the pleon bears about 15 setae instead 



of only 4 in front of the articulation of the uropoda. 



(4) The first pair of legs in both sexes differ slightly in the shape of the propodos and 



in the armature of the palm. 

 (6) Thei"e are also slight differences in the maxillipedes. 

 (6) The body is much stouter than in either of the subterranean species. 



Without a much fuller knowledge of the habits of each species than we possess, it is 

 difficult to see the reason for the differences between them ; and until we are able to do 

 this, to some extent, it will be almost impossible to assign its true importance to each 

 difference and thus to discover the true relationships of the species. The abundant setae 

 on P. austraUs are perhaps protective ; and, if so, we can see why they should be less 

 abundant in the subterranean forms, though even in these species they are pretty 

 numerous, especially on the last three pairs of legs. The slender body and appendages 

 of P. typicus may also be an adaptation to a subterranean mode of life, and, if so, it 

 would appear that P. typicus has been longer underground than P. assimilis ; but in the 

 present state of our knowledge all speculations of this kind must be received with the 

 greatest caution. The questions suggested may perhaj)s be some day solved by the 

 discovery of species of Plireatoicus still living above ground in the mountain-streams of 

 the Southern Alps, places where very little search of the kind required has hitherto 

 been made. 



It is worthy of notice that the species of Plireatoicus do not show the increase in the 

 number of sensory setae, &c., in compensation for the loss of eyes that has been observed 

 in some other subterranean species. (See p. 262.) 



Special points in the Structure of Phreatoicus. 



In many respects Phreatoicus appears to be a very generalized type of the Isopoda^ 

 possessing all the segments of the body and their appendages in a more perfect form than 

 any other Isopod I know. Thus in the body all the segments both of the jieraeon and 

 the pleon are well developed and separate, except of course that the telson is joined to 

 the sixth segment of the pleon as in nearly all Isopocla. The antennae, thougli well 

 developed, do not jjresent any peculiarity, and the lower antenna does not possess the 

 rudimentary exopodite found in some genera of the Asellidae, such as Janira, lanthe, 

 Stenetrium [9, p. 9]. The mouth-parts are all particularly well developed, no parts 

 usual in the Isopoda being absent or coalesced ; the maxillipedes especially have all the 

 joints pei'fect and separate. The legs of the peraeon all have the coxae more or less 

 separate from the segment, showing, 1 think, clearly that they are really the first joints 

 of the legs, and not outgrowths of the body-ring (" epimera "). This view was first 

 advanced by Spence Bate in 1855 [7], and has, I believe, since been pretty generally 

 adopted, though, as Stebbing says, " It is a disputed question whether we have at the 

 base of the leg an outgrowth of the body-ring carrying the more or less obsolescent first 

 joint of the leg soldered to it, or whether the $ide-plate is itself a protective expansion of 

 the first joint " [108, p. 17bO]. In quoting Spence Bate's arguments to show that the 



