ARTHROPODA. 
283 
ages. The five segments of the head are compressed into 
a very small space, yet have the following members : 1473 
the short and the long antennae; the mandibles, or jaws, 
between which the mouth opens; and the two pairs of 
maxillae. The thorax carries three pairs of modified limbs, 
called “foot-jaws,” and five pairs of legs. The foremost 
legs, “the great claws,” 
are extraordinarily de¬ 
veloped, and terminat¬ 
ed by strong pincers 
( chelcB ). Of the four 
slender pairs succeed¬ 
ing, two are furnished 
with claws, and two 
are pointed. The last 
pair of swimmerets, to¬ 
gether with the telson, 
form the caudal fin— 
the main instrument of 
locomotion; the others 
(called “swimmerets”) 
are used by the female 
for carrying her eggs. 
The eyes are raised on 
stalks so as to be mov¬ 
able (since the head is 
fixed to the thorax), 
and are compound, 
made up of about two 
thousand five hundred square facets. At the base of each 
small antenna is a minute sac, whose mouth is guarded by 
hairs: this is the organ of hearing. The gills, twenty on- 
a side, are situated at the bases of the legs and enclosed in 
two chambers, into which water is freely admitted, in fact, 
drawn, by means of a curious attachment to one of the 
Fig. 250. —Under-side of the Cray-fish, or Fresh¬ 
water Lobster {Astacus fluviatilis): a, first pair 
of antennae; 6, second pair, c, eyes; cl, open¬ 
ing of kidney; e, foot-jaws ; /, g, first and fifth 
pair of thoracic legs; h, swimmerets; i, anus; 
k, caudal fin. 
