106 STRUCTURE OF ARACHNID A AND CRUSTACEA. 
tained in these cavities. At c t is seen the cephalo-thorax 
opened from below, and giving attachment to the legs; at m 
is shown the place of the mandibles or jaws; at p is seen one 
ct pa ab pa s 
p pa t a l s ma o f 
Fig. 46.—Anatomy of Spider. 
of the palpi, which are appendages to the mouth; pa is the 
foremost leg; t , the large nervous mass, from which the legs 
are supplied; a , the collection of ganglia supplying the 
abdomen; a b, the abdomen; p a , the respiratory chambers ; 
s s , the stigmata or openings into these; l, the leaf-like folds 
within them (§ 323); m $, the muscles of the abdomen; a n, 
the termination of the intestine; /, the spinnerets; o, the 
ovaries; and o r, the opening of the oviduct. 
99. The class of Crustacea, of which the Crab , Lobster , 
and Cray-fish are the best-known forms, differs from both 
the preceding, in being adapted to breathe by means of gills, 
and thus to reside in or near water, instead of inhabiting 
the air. Moreover, the body is inclosed in a hard covering, 
which generally contains a good deal of carbonate of lime, and 
which is thrown off at regular intervals. This covering also 
incloses the members, which are never less than ten in 
number, and are frequently more numerous. There is great 
variety of form among the animals of this group, which is 
altogether one of great interest.—In the Crab tribe, the head, 
thorax, and abdomen are all drawn together, as it were, into 
one mass; and the general arrangement of the organs it con¬ 
tains is exhibited in the succeeding figure, which shows them 
nearly as they are found to lie, when the upper part of the 
shell, or carapace , is removed. At t there is left a portion of 
