130 COMPARATIVE ZOOLOGY. 
are pierced with small holes, through which the animal 
sends out fleshy tentacles. 
The skin of the Crab and Lobster is hardened by cal- 
careous deposit into a “crust,” or shell ;* but, instead of 
forming one piece, it is divided into a series of segments, 
which move on each other. The number of these seg- 
ments, or rings, is usually twenty-one—to the head, tho- 
rax, and abdomen, seven each. In the adult, however, 
the rings of the head and thorax are often soldered to- 
gether into one shield, called cephalo-thorax ; and in the 
Horseshoe Crab all the divisions of the skeleton are quite 
obliterated. The shell of Crustaceans is periodically cast 
off, for they continue to grow even after they have reach- 
ed their mature form. This molting is a very remarka- 
ble operation. How 
the Lobster can 
draw its legs from 
their cases without 
unjointing or split- 
ting them has long 
been a puzzle.™ 
But the cast-off 
skeleton is a per- 
fect copy of the 
animal, retaining in 
their places the del- 
icate coverings of 
the eyes and an- 
tennze, and even the 
lining membrane of 
Fig. 98.—Diagram of an Insect: A, head bearing the the stomach with 
eyes and antennz; B, prothorax, or pronotum, car- ; 
rying the first pair of legs; C, mesothorax, carrying its teeth! 
the second pair of legs and first pair of wings; D, . ’ 
carrying the third pair of legs and second pair of The hor ny ust 
wings ; E, abdomen, with ovipositor, F; 1, coxa, or of Insects differs 
hip; 2, trochanter; 3, femur, or thigh; 4, tibia, or 
shank; 5, tarsus, or foot; 6, claw. from that of Crus- 
