350 



ARTHROPODA 



merited by a ventral chain composed of ganglia metamerically arranged. 

 The most evident distinctions between the annelids and the arthropods 

 are (i) the character of the segmentation and (2) the presence of jointed 

 appendages. 



In superficial appearance the lines between the segments are con- 

 stricted more deeply in the arthropods than in the annelids. The cause 

 of this lies in the character of the integument (fig. 26, f), 

 which is developed as a hard armor, in which two 

 layers are recognizable, the epidermis ('hypodermis') 

 and the chitinous layer. The epidermis is a thin 

 epithelium, while the chitinous layer is of greater 

 thickness and, since it is secreted by the epidermis, is 

 stratified parallel to the surface. Its firmness is due 

 to chitin, which is unlike most organic substances in 

 its resistance to acids and alkalis; only under the 

 action of sulphuric acid and heat is it broken up into 

 sugar and ammonia. Frequently (Myriapods, Crus- 

 tacea) the chitinous armor is strengthened by the 

 deposition of calcium carbonate and phosphate. A 

 firm coat would render the animal incapable of motion 

 were there not joints between the parts. While the 

 segments 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 impossible (fig. 364). 



Since the ringing of the body is connected with this armoring, it disappears 

 with the need for such protection. The hermit crabs (fig. 406) illustrate this. 

 These animals live with the abdomen inserted in a snail shell. That part of 

 the body which projects from the shell is armored, while the abdomen is soft- 

 skinned and without traces of external ringing. The hardened cuticula causes 

 the periodic molting (ccdysis or exuviation } . When once hardened it is incapable 

 of distention and so would prevent farther growth. Hence when the body has 

 completely filled the shell, the latter splits in definite places and the animal crawls 

 out of the old 'skin' (exui'ia) and rapidly increases in size, while the new cuticula 

 is yet soft and distensible. 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 base 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. 80). The sense 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 system. The cell itself may be in the epithelium or situated deeper and 

 interpolated as a ganglion cell in the sensory nerve. 



The muscles which are inserted on the integument are sesrmental in character 



FIG. 364. Dia- 

 gram of Arthropod 

 jointing; .4, in ex- 

 panded, B, in con- 

 tracted condition; 

 1-4, rings with con- 

 necting membranes, 

 the muscles indi- 

 cated by dotted 

 lines, (after 

 Graber). 



