ooO 



ARTlIROrODA 



mented liv a ventral chain composed oi ganglia mctamcrically arranged. 



The most e\ident dislinclions liet\\-ecn the annelids and the arthropods 



are (i) the character of the segmentation and {^) the presence of joiitted 



appendages. 



In superficial appearance the lines helween tJte segnienls are con- 



stricteil more deeply in lite arthropods than in the annelids. The cause 

 of litis lies in the character of the inieguntent (tig. .vi, /'), 

 wlticlt is developed as a Itard armor, in which two 

 lavers are rccogitizahle, t.he e]>iderntis (■h>podermis') 

 and the cltilinous layer. Tlte epiilermis is a lltin 

 epithelium, wliile the chitinous la}-er is of greater 

 iliickness and, since it is secreted by ihe epidermis, is 

 stralilied parallel to the surface. Its lirmness is due 

 to c'lil'ni, wliich is unlike most organic sulislaitces in 

 its resistance to acids and alkalis; only under the 

 action of sulphuric acid and heat is it broken up into 



gram'of AriliTopoJ ^ugar and ammonia. Freciuently (Myriapods, Crus- 



jointing; .t, in ex- tacca) the cliitiuous armor is strengthened l>y the 



panded, B. in 

 traded comiilioii ; 

 1-4, rings with con- 

 necting membranes, 

 tlie muscles inili- 

 cated by dotted 

 lines, (, a '' 1 1^ r 



G ruber). 



deposilioit of calcium carbonate and phosphate. A 

 firm coat would render tlte animal incapable of motion 

 were there not joints between tlte parts. \Mtile ihe 

 segments themsel\es are heavily armored, the cuticle 

 between them is redticed to a delicate articular skin, 

 and this is so protected by a kimi of telescoping of the 

 segments that injury in these softer regions is itearly impossible (lig. 304). 



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

 with the need for such protection. The hermit crabs (lig. 40O) illustrate this. 

 These animals li^■e 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 (cidysis or iwiivialiini). \\ hen once hardened it is incajtable 

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

 completely tilled the shell, the latter splits in dehnile places and the animal craw Is 

 out of the old 'skin' (c.xiivia) and rapidly increases in size, while the new cuticula 

 is yet soft and distensible. Another restilt of the cuticula is seen in the peculiar 

 relations of both ordinary and sense hairs. These are culicular 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 surrouniling chitin, and hence 

 is movable; it is traversed by a canal in which is a process of the underlying 

 matri.x cell. In the case of sensory hairs these structures are connected with a 

 nerve (fig. So). The sense cell has two processes; one perijiheral, which enters 

 the axis of the hair, the other ceiUral, which runs as a nerve hbre to the central 

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

 interpolated as a ganglion cell in Ihe sensory ner\e. 



The nniscles w hich are inserted on the integument are segmental in character 



