162 THE SHORTER SCIENTIFIC PAPERS 



aerial respiration through the development of the integu- 

 mentary pockets as tracheal openings, compared to that 

 which has taken place in Peripatus and allied genera, are 

 infinitely less than the changes that have occurred in the 

 descent of the terrestrial vertebrates from aquatic forms. 



There are many of the existing Polychaetes in which 

 the structure of the parapodium is closely in accord with 

 the form noted and to which attention will be called in a 

 subsequent part of the present paper. 



In turning to a discussion of the external structure of 

 the insects, let us first consider a typical segment with 

 reference primarily to the chitinous skeletal system, for 

 it is here that the relationship with the Polychaetes is most 

 apparent. The segment consists of a dorsal area {tg ^ 

 tergum), a lateral area (pi ^= pleurum) and a ventral 

 area {si = sternum). The pleural area is formed from 

 two pieces, of which the one (eps = episternum) occupies 

 an anterior ventral position, while the other {epvt = 

 epimeron) is found on the posterior dorsal part of the 

 pleurum. The appendages are the leg formed from the 

 "coxa," which in turn is composed of an anterior ventral 

 piece (c = coxon) and a posterior dorsal piece (m = 

 meron), and an evaginated sack-like part of the integu- 

 ment forming the wing {pt = pteron). Anterior to the 

 wing is a sclerite (pr = propteron) which from its 

 tracheation in certain cases has apparently developed in 

 a way similar to the wing, and which from its constancy 

 in the various orders of insects is here included as a typical 

 part of the segment. A large seta-like appendage (s = 

 stylus) is attached to the meron (Machilidae) of the 

 meso- and meta-coxae as well as to the sternal abdominal 

 coxae in many of the Apterygotes. Tillyard advises me 

 that the stylus is segmented in some of the New Zealand 

 Machilids. On the anterior margin of the pleural area is 

 the integumentary opening (sp = spiracle) of the trachea. 



Many secondary areas have developed in the insects 

 and the groups allied to them. Among these are to be 

 noted the trochantin, ante-coxal piece, subdivisions of the 

 sternum, tergum, episternum, epimeron, propteron, etc. 

 Included in such areas I would place the micro- thorax of 



