226 



MANUAL OF ZOOLOGY. 



legs are expanded so as to form a swimming- tail. Some of 

 this section are parasitic upon various fishes {Cymothoa), whilst 

 others are found in the sea {Sphcsroma). In the Sedentary 

 Isopoda the animals are all parasitic, with short, incurved, 



hooked feet. This section 

 includes the single family of 

 the Bopyridce, all the species 

 of which live parasitically 

 either in the gill-chambers, 

 or attached to the ventral 

 surface, of certain of the 

 Decapod Crustacea, such as 

 the Shrimps ( Crangones) and 

 the Palamones. 



The Cursorial, or run- 

 ning, Isopods mostly live 

 upon the land, and are 

 therefore destitute of swim- 

 ming-feet. The most famil- 

 iar examples of this section 

 are the common .Wood-Uce 

 (Oniscus). Here, also, be- 

 longs the little Limnoria 

 terebrans, so well known for 

 the destruction which it pro- 

 duces , by boring into the 

 wood-work of piers and other structures placed in the sea. 

 Other well-known Isopods are the Water-slaters {Asillus) of 

 fresh waters, the Rock-slaters {Ligid) of almost all coasts, the 

 Box-slaters {Idothea), the Shield-slaters {Cassidina), and the 

 Cheliferous Slaters {Tanais). These last are remarkable as 

 being the only Isopods in which there is a carapace. The 

 lateral parts of the carapace are highly vascular, and respira- 

 tion is effected by these, and not by the abdominal feet. 



Many Isopods undergo an extensive metamorphosis. " In 

 some Fish-lice {Cymothoa) the young are lively swimmers, and 

 the adults are stiff, heavy, stupid fellows, whose short clinging 

 feet are capable of little movement." In the Bopyridce the 

 adult females are usually blind, the antennae are rudimentary, 

 and the abdominal appendages from natatory become respira- 

 tory organs. The males, on the other hand, are dwarfed, and 

 sometimes lose all the abdominal appendages, and all traces of 

 segmentation ; until we get forms which, like Cryptoniscus pla- 

 narioides, "would, be regarded as a Flat-worm rather than an 

 Isopod, if its eggs and young did not betray its Crustacean 

 nature." — (Fritz Miiller). 



Fig. 73- — Isopoda. Wood-lice {Oniscus). 



