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 

 cuticle between them is reduced to a delicate 

 articular skin, and this is so protected by a 

 kind of telescoping of the segments that 

 injury in these softer regions is nearly im- 

 possible (fig. 399). 



Since the ringing of the body is connected with 

 this armoring, it disappears with the need for 

 such protection. The hermit crabs (fig. 480) are 

 instructive illustrations of this. These animals live FIG. 399. Diagram of Ar- 



., , rr,, thropod jointing ; A, m 



with the abdomen inserted in a snail shell. 1 hat expanded, B, in con- 

 part of the body which projects from the shell is S^tiT^nectin'g 



armored, while the abdomen is soft-skinned and membranes, the mus- 

 . . . cles indicated by dotted 



without traces of 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 the 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 which is a process of the 

 underlying matrix cell. In the case of sensory hairs these 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 the hair, 

 the other central, which runs as a nerve fibre to the central nervous sys- 

 tem. The cell itself may be in the epithelium or situated deeper and 

 interpolated as a ganglion cell in the sensory nerve. 



Another important character is the heteronomous segmentation, 

 which, in the lowest forms (Peripatus and Myriapods), is little 

 pronounced, but elsewhere leads to a marked inequality of the 

 divisions of the body and to a greater centralization of structure. 

 Different body regions may be distinguished. A few segments at 

 the anterior end always fuse and form a head (fig. 400, 0) ; behind 

 this there is usually a second segment complex, the thorax (T), and 

 then a third, the abdomen (^4). An apparent reduction of regions 

 can occur when the head and thorax unite (fig. 401, Ct) to form 

 a cephalothorax ; or again the number of regions may be increased 

 (fig. 402) by a division of the abdomen into abdomen proper (A) 



