ISOPODA. 
2 75 
Sphceruma (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 
brinof about is due to their habit of boring 1 into timber below 
o o 
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 
Cymothoidce 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). 
ms 
male gnathia (enlarged). 
