. A manual of zoology. Zoology. ARTHROPODA. 399 A firm chitinous armor would render the animal incapable of motion were there not joints between the parts. While the seg- ments themselves are heavily armored, the » e cuticle between them is reduced to a delicate articular skin, and this is so protected by a kind of telescoi)ing of the segments that injury in these softer regions is nearly im- possible (fig. 3911). Since the ringing of the body is connected with this armoriug, it disappears with the need for such protection. The hermit crabs (fig. 480j are instructive illustrations of this. The
. A manual of zoology. Zoology. ARTHROPODA. 399 A firm chitinous armor would render the animal incapable of motion were there not joints between the parts. While the seg- ments themselves are heavily armored, the » e cuticle between them is reduced to a delicate articular skin, and this is so protected by a kind of telescoi)ing of the segments that injury in these softer regions is nearly im- possible (fig. 3911). Since the ringing of the body is connected with this armoriug, it disappears with the need for such protection. The hermit crabs (fig. 480j are instructive illustrations of this. These animals live Fig. 399.—Diagram ofAr- .,, .. , -, • i 1 • -1 1 n rri J- thropod jointing; ^4, in witli the abdomen inserted m a snail shell. That expa^ided, B. in con- part of the body which projects from the shell is *™^'/iiX'^J*n"n"eitint armored, while the abdomen is soft-skinned and membranes, the mus- ... , ^ J, . ... clesindicated by dotted Without external ringing. lines. (After Graber.) The hardened cuticula causes the periodic molting (ecdysis or exuvia- tion). When once hardened it is incapable of distension and so would prevent farther growth. Hence when tlie body has completely filled the armor, the latter splits in definite places and the animal crawls out of the old ' skin' (exuvia) and rapidly increases in size while the new cuticula is yet soft and extensible. Another result of the cuticula is seen in the peculiar relations of both ordinary and sense hairs. These are cuticular structures, each usually secreted by a single epidermal cell and renewed after each molt. Each hair has a ball-like head situate in a socket in the surrounding chitin, and hence is movable ; it is traversed by a canal in wdiich is a process of the underlying matrix cell. In the case of sensory hairs those structures are connected with a nerve (fig. 77). The sense cell, like a bipolar ganglion cell, has two processes ; one peripheral, which enters the axis of
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