42 



Guide to Crustacea. 



Table-case This is a very large and varied group, comprising numerous 

 No. 6. families which are grouped under six Sub-orders. 



In the Sul3-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 Aselhis 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 Flabellifera 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 Gymotlioidac, are 

 blood-sucking parasites of fish, and some of 

 them are remarkable for being hermaphrodite 

 (like the Cirripedia), each animal being at first 

 a male and afterwards a female. This family 

 includes the giant of the Order, the deep-sea 

 Bathynomus gigantcus (Fig. 22), which some- 

 times reaches an even greater size than the 

 specimen exhibited. 



A contrast in point of size is provided by 

 the minute Limnoria lignoruvi (Fig. 23), belong- 

 ing to the family Sphaeromidac, 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 l:)y the " White 

 ants " of tropical countries. A good example of the results of 

 their activity is given by a piece of timber from Eyde pier 

 exhibited in Wall-Case No. 4 (Fig. 24). 



The Sub-order Valvifeka is characterised l)y tlie fact that the 

 uropods form a pair of plate-like " valves " closing over the 

 remaining five pairs of abdominal appendages. This Sub-order 



Limnoria lignonnn, 



much enlarged. 



(After Sars.) 



[Table-case No. 6.] 



