208 DE. C. CHILTON OX THE SUBTEERAXEAN 



of the Asellidae thaa to those of any other Isopods that I know. Besides differing 

 very greatly in the shape of the body, Phreatoicus differs greatly from the Asellidse, as 

 from the Idoteidse, in the structure of the pleon. In the AselUdae this is short, usually 

 composed of a single flattened piece, and the pleopoda lie horizontally under it, and are 

 protected by a more or less perfect operculum formed of the first pair. In Fhreatoiciis 

 the segments of the pleon are all separate, and the pleopoda hang vertically down, and 

 are not protected below ; indeed the shape of the pleon renders protection of the pleopoda 

 below unnecessary. 



These differences are pretty considerable and quite enough to show that Phreatoicns 

 cannot be placed under the Asellidae, but they are of such a nature that they do not 

 prevent us from considering that the affinities of Phreatoicus ave with the Asellidse. For 

 it is quite clear that the latter must have arisen from ancestors possessing a pleon formed 

 of six separate segments, and that these have gradually coalesced to form a single plate ; 

 just as we see the same process going on at the present time in the Idoteidae, where some 

 species have the pleon formed of four or five segments, others of only two or three, 

 and others, again, like Idotea elouc/ata, Miers, with the pleon formed of a single piece 

 [2I'l, p. 198]. The horizontal position of the pleopoda and the development of an 

 operculum from the first jjair would naturally follow from the flattening of the body in 

 the Asellidae, which would otherwise leave the pleopoda much exposed below. Thus 

 Phreatoicus appears to differ from the Asellidse chiefly in having preserved the fully- 

 developed pleon which must have been possessed by the ancestors of the Asellide, while 

 in the latter this has been specially modified in accordance with the general flattening of 

 the body, which would render a long-jointed pleon unsuitable and a source of danger to 

 the animal, especially by the exposure to w^hich it would subject the pleopoda. The 

 flattening of the body in the Asellidse would naturally follow as the result of their 

 adopting a creeping mode of life ; Phreatoicios walks erect or swims much in the same 

 way as the Amphipoda. 



There is one genus, Llmnoria, formerly classed with the Asellidse, which differs from 

 them and resembles Phreatoicus in having the pleon composed of six separate segments 

 with the pleopoda unprotected. Limiioria, however, resembles the normal Asellidse in the 

 flat depressed body, and the segments of the pleon, though separate, are short, and it may 

 perhaps be looked upon as an approach toward the ancestral form of the Asellidae, 

 though its structure has been modified to some extent to suit its mode of life ; thus the 

 antennae are very short, and the legs are short and perhaps little used for walking, and 

 the mouth-parts are somewhat modified. Unfortunately, I do not know sufficient of the 

 pleopoda of Linmoria to compare them with those of Asellus and Phreatoicus, but from 

 the other resemblances we may with good reason look upon Limnoria as an intermediate 

 link, to some extent connecting Phreatoicus with the AseUidae. The very great difference 

 in appearance between the two latter is due to the fact that the body of the Asellidse is 

 flat, depressed, and the animals are therefore represented as seen from above, while, 

 owing to its body being somewhat laterally compressed, Phreatoicus is usually seen in 

 side view. This difference in the form of the body is, however, probably not of much 



