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A much more important difference between Phreatoicus and the 
Idoteidce is however to be found in the abdomen and particularly 
in the last pair of appendages — the uropoda. In Phreatoicus these 
are fairly normal in character and not very dissimilar from those 
found in Asellus and other genera, while in the Idoteidm they are 
specially modified into flat subrectangular plates to cover the lower 
surface of the abdomen and to protect the pleopoda. In some 
species of the Idoteidcr , however, there is a rudimentary second 
ramus, showing that this plate is formed from the typical uropod 
consisting of a basal joint and two rami, the inner ramus being 
rudimentary or absent, and the formation of the protecting plate 
is probably a special modification of comparatively recent date. On 
the whole the affinities of Phreatoicus to the Idoteidce may be 
considered to be somewhat greater than its affinities to the 
Anthuridce. In the same way we might compare Phreatoicus with 
the Arcturidce , a group that must be placed near the Idoteidw. 
When we come to compare Phreatoicus with the Asellidve we 
see at once that the form of the body is very different from that 
usually found in the Asellida », but on examining in detail we find 
that there is a fairly close resemblance in many other respects. 
The head, antenna* and mouth parts are all in close agreement, 
except that the head is flattened in the Asellidre ; the resem- 
blance in the mouth parts is on the whole very close and is 
somewhat striking, considering the great difference in the general 
appearance of the animal. 1 have been able to take Sars’ descrip- 
tion of the mouth parts in Aselhus aquations as my guide in 
describing those of Phreatoicus , and to follow that guide very 
closely. All the legs and the uropoda are also closely similar, 
the resemblance in some of the legs being quite as close as that in 
the mouth parts. It is in the abdomen and the pleopoda that we 
find the greatest difference between the two. In the Asellidce the 
abdomen is usually depressed and formed of a single piece, though 
traces of other segments are sometimes present, the pleopoda lie 
closely under the abdomen and are usually protected by a more 
or less perfect operculum formed of the first pair of pleopoda. 
In Phreatoicus the abdomen is not depressed but somewhat com- 
pressed laterally, the six segments are all separate and well 
developed, the pleopoda hang vertically downwards from the 
segments of the abdomen and there is no operculum, the pleopoda 
being exposed below though protected laterally by the pleura of 
the abdominal segments. 
These differences in the abdomen are pretty considerable, but 
from the traces of separate segments found in some of the species 
of the Asellida} it is quite clear that the ancestors of the Asellidce 
possessed an abdomen 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 Idoteidce , where 
