18 COMPARATIVE ANATOMY. 



SUB-KINGDOM ARTHROPODA. 



Division TRACHEATA. 



Class MYRIAPODA. 



29. A Centipede, Scolopendra, a Chilopodous Myriapod. 



The segments of the exo-skeleton differ from those of In- 

 secta and Crustacea in not being definitely marked out into 

 cephalo -thoracic, abdominal, and post-abdominal regions. There 

 are twenty- three post-cephalic segments, to each of which a 

 single pair of appendages is attached. Any one of these append- 

 ages consists of a dorsal and ventral arc, united together late- 

 rally by the soft integument from which the appendages take 

 origin. On either side of the mesial line of the ventral arc are 

 two very faint lines, which mark off a median portion, or 'ster- 

 num/ from two lateral elements, or 'episterna.' These faint 

 lines are best seen in the sterna of the anterior segments. Im- 

 bedded in the lateral integument are one or two chitinous plates 

 or tubercles, representing the rudimentary 'epimera' or lateral 

 elements of a typical 'somite.' Each appendage is many-jointed. 

 The proximal joint is the 'coxa,' and the distal one carries a 

 sharp curved claw. Each segment is to be regarded as being 

 composed of two sub-segments, which at first are equally deve- 

 loped, but as development proceeds, one of them increases in 

 size more rapidly than the other; and moreover develops a pair 

 of appendages which the other does not. .This latter, the 

 anterior one, becomes abortive, though in the Chilognatha it 

 develops equally with the posterior one, with which it event- 

 ually coalesces, and moreover carries a pair of appendages. 

 Nevertheless in this specimen the remains of the anterior sub- 

 segment are to be seen. Immediately in front of each sternal 

 piece, and on each side of the mesial line, are three small 

 angular chitinous plates. The pair nearest the mesial line are 

 the two uncoalesced halves of the sternum of the aborted anterior 

 sub-segment. The pair external to these are the episterna, 

 and the most external and smaller pair are the epimeral plates. 

 Epimeral plates are also situated, one pair on the hinder edges 

 of the sternum, and two small plates just in front of the coxa, 

 imbedded in the lateral integument. The two sub-segments, 



