CARNIVOROUS CRUSTACEA AND KING-CRABS 



143 



curious " Whale-lice " found as parasites on the skins of Cetaceans 

 are related to the Skeleton-Shrimps, though of very different 

 proportions. 



IsoPODS. — Probably the best-known member of this group to 

 ordinary observers is the Wood- Louse {Armadillidium vulgare), 

 which rolls itself into a ball when disturbed; but as this and its 

 terrestrial brethren are largely or mainly vegetarian in habit, 

 they require no further mention here. Although by no means 

 all of the marine forms are carnivorous, many of them are 

 markedly so, and this is the case with the familiar "slaters" 

 found lurking in rock -crevices between tide -marks. The Sea- 

 Slater {Ligia oceanica) (fig. 405), e.g., and the species of the 

 shallow-water genus Idotea, are pre- 

 daceous. Other examples are found 

 in the species belonging to the genus 

 Cirolana (see vol. i, p. 415), for which, 

 as in so many other cases, only scien- 

 tific names can be given. Speaking 

 of one of these species (C borealis) 

 Stebbing says (in A History of Crus- 

 tacea): — "It is a good swimmer, tena- 

 cious of life, a savage devourer of fish, 

 and not to be held in the human hand 

 with impunity ". An allied American 

 species preys upon an edible variety 

 of crab. As may be imagined, well- 

 developed biting jaws are present in 

 such rapacious forms as those just 

 mentioned. The marine Isopods also 

 include a remarkable series of forms 

 parasitic upon fishes and higher Crus- 

 tacea. Some of these are so remarkably degenerate, as a result 

 of their dependent mode of life, that they have lost all outward 

 resemblance to their free-living relatives. 



The group has also freshwater species, of which one, the 

 Water Wood- Louse {Asellus aquaticus), is often to be found 

 among rotting vegetation, feeding upon both animal and vegetable 

 substances. 



Fig. 405. — Sea-Slater [Ligia oceanica^ 



