42 
Guide to Crustacea. 
Table-case 
No. 6. 
This is a very large and varied group, comprising numerous 
families which are grouped under six Sub-orders. 
In the Sub-order Asellota the uropods are slender ; the basal 
segments of the legs are not coalesced with the body as in most 
other Isopoda ; the first pair of abdominal limbs are generally 
fused, in the female, to form an operculum, or cover for the 
remaining pairs. This group includes Asellus aquaticus, which is 
common everywhere in ponds and ditches in this country, and a 
very large number of marine species, mostly of small size. 
The Sub-order Phreatoicidea includes a small number of very 
peculiar species found in fresh water in Australia and New 
Zealand. In these the body is flattened from 
side to side, and the animals in other respects 
have a superficial resemblance to Amphipoda. 
In the Sub-order Plabellifera the ter- 
minal limbs of the abdomen (uropods) are 
spread out in a fan-like manner on each side 
of the telson. Many species of this group, 
belonging to the family Cymothoidae, are 
blood-sucking parasites of fish, and some of 
them are remarkable for being hermaphrodite 
(like the Oirripedia), each animal being at first 
a male and afterwards a female. This family 
includes the giant of the Order, the deep-sea 
Bathynomus giganteus (Fig. 22), which some- 
times reaches an even greater size than -the 
Limnoria liqnorum, • ^ -t i 
much enlarged. specimen exhibited. 
(After Sars.) A contrast in point of size is provided by 
[Table-case No. 6.] m i nu t e Limnoria lignorum (Fig. 23), belong- 
ing to the family Sphaeromidae, which, 
however, forces itself upon human attention by reason of its 
destructive powers. In company with a member of the next Order, 
the Amphipod Chelura terebrans, it burrows in submarine timber, 
and by their enormous numbers the two species often destroy 
the piles of jetties and such-like structures to an extent which is 
only paralleled by the havoc wrought on land by the “ White 
ants ” of tropical countries. A good example of the results of 
their activity is given by a -piece of timber from Ryde pier 
exhibited in Wall-Case No. 4 (Fig. 24). 
The Sub-order Valvifera is characterised by the fact that the 
uropods form a pair of plate-like “ valves ” closing over the 
remaining five pairs of abdominal appendages. This Sub-order 
