CRUSTACEA, 143 
is not only extremely short, but it is always tucked up beneath 
the greatly-developed cephalothorax, so that it is not visible at 
all, except when the animal is looked at from below (fig. 64). 
The crabs are very various in their habits, but they are mostly 
denizens of the shore, hiding beneath stones or sea-weed, in 
cracks of rock, or in pools near the line of low-water. Some of 
them, however, can swim with tolerable activity, and some of 
them (the land-crabs) even live habitually inland. One group, 
that of the “pea-crabs,” is distinguished by the singular habit 
of living semi-parasitically within the shells of bivalve molluscs, 
such as the great horse-mussel. 
The young or larval crab is exceedingly unlike the adult, 
and has a long and well-developed abdomen, thus approximat- 
ing to the type of structure which is permanently retained in 
the Macrura. 
ORDER Isopopa.—In this order are a number of Crustaceans 
of which some inhabit the 
sea, others are parasitic in 
their habits, and others are 
terrestrial. The best known 
are the common wood-lice 
(Oniscus, fig. 65), which 
are found so commonly 
under stones, or in the 
crevices of old walls. The 
Isopods all belong to a 
group of Crustaceans in 
which the eyes are not sup- 
ported upon stalks, and 
they are therefore said to 
be “sessile-eyed.” The | 
head is distinct from the 
segment bearing the first 
pair of feet. The thoracic 
feet are all similar to one 
another, and the branchiz 
are developed on the ab-_ Fig. 65.—Isopoda. Wood-lice (Oniscus.) 
dominal legs. 
ORDER MEROSTOMATA,—In this order are only the living 
