ISOPODA. 



75 



Sphceroma (enlarged). 



dissimilar that they were referred to two families. In the adult male the 

 mandibles are powerful and prominent, and the head is large, squared, and at 

 least as wide as the thorax. In the adult female, on the con- 

 trary, the head is small and triangular, without visible man- 

 dibles, and the thorax is much dilated. Many species are known 

 from the European coasts, and one has been obtained at a depth 

 of nine hundred fathoms. Belonging to this tribe, but repre- 

 senting a family by itself, is Limnoria lignorum, known to 

 fishermen as the gribble, which is a persistent destroyer of 

 submerged wood. The creatures are about one-sixth of an inch 

 long, and of an ashy grey colour ; and the destruction they 

 bring about is due to their habit of boring into timber below 

 water-mark. They are vegetarians, and feed on the wood which 

 they excavate. The members of the group known as fish-lice 



are mostly of large size, the body being longish and 

 oval, and the antennae fixed on the front of the head, 

 which bears in addition two large eyes. The anterior 

 three pairs of thoracic limbs are stout and prehensile, 

 terminating in strong curved claws, while the posterior 

 four pairs are longer and thinner, and adapted for 

 crawling. By means of their powerful fore-feet the 

 CymothoidcB attach themselves to both marine and 

 fresh-water fish, and have a liking for the inside of 

 the mouth of their hosts. 



Another tribe is the Epicaridea, the members of 

 which live parasitically upon other crustaceans. The 

 form of the body in the female is, as a rule, distorted 

 and unsymmetrical ; but the smaller males are sym- 

 metrical, and are usually found adhering to the females. 

 No group of Crustaceans seems exempt from the attacks 

 of these parasites, but it is said that each species has its peculiar kind. 



The best known example of the tribe Asellota is 

 Asellus aquaticus, distributed in fresh- water ponds and 

 ditches almost all over Europe The creature is of a 

 greyish colour, mottled with paler markings ; and the 

 male, which is longer than the female, measures about 

 half an inch long. The body is long, narrow in front, 

 with a small head, and the antennae of the second pair 

 are about as long as the body and head taken together. 

 The seven segments of the thorax are free and of large 

 size, but those of the abdomen have coalesced into a 

 plate, from the end of which the long slender forked 

 uropods project. The seven thoracic limbs are long, 

 slender, and increase in length from the first to the 

 seventh. 



The tribe Oniscoidea contains the wood-lice, in female gnathia (enlarged). 



male gnathia (enlarged). 



