160 THE SHORTER SCIENTIFIC PAPERS 



suited from the development of a ridge which was needed 

 to strengthen the pleural area as a support to the leg. 

 The more recent papers devoted to the anatomical struc- 

 ture of the insects by Berlese, Borner, Bugnion, Cramp- 

 ton, Feuerborn, Franz, Goldi, Handlirsch, Heider, Hey- 

 mons, Imms, MacGillvray, Verhoeff, Weber and others 

 present no new interpretations. 



In this paper it is my purpose to call attention to some 

 of the evidence that has induced me to believe that the 

 ancestral type from which the insects and their allies 

 have developed belongs to the Polychaetes, and that in- 

 termediate forms will eventually be found in the rocks of 

 the Cambrian or Silurian periods. The well-preserved 

 fauna of the Burgess Shales belonging to the Middle 

 Cambrian of British Columbia, which Walcott has noted 

 in a series of papers, is of decided interest in this connec- 

 tion. 



The general argument as to the Polychaete ancestry of 

 the insects may be stated most advantageously in connec- 

 tion with a series of diagrams illustrating the course that 

 evolution has taken, on the basis of the hypothesis ad- 

 vanced. 



First, let us consider the structure of the external parts 

 in a typical segment of a living Polychaete. The epi- 

 dermal cells underlying the integumental covering have 

 secreted a thin protective cuticula with no pronounced 

 folding except in connection with the slightly marked 

 secondary annuli, which at times are marked off by 

 transverse furrows. The latter development of a pro- 

 nounced segmentation, as it takes place in the various 

 groups descended from the annelids, closely parallels the 

 phyletic development of the notochordal area of the ver- 

 tebrates, which like the insects and their ancestors have 

 passed from an aquatic to a terrestrial environment. 



In the Polychaetes the striking characteristic of the 

 segment is the development upon each side of a lateral 

 fold called the parapodium, which propels the organism 

 through the water and along the sea-bottom. It is at- 

 tached in a plane nearly perpendicular to the longitudinal 

 axis of the body and consists of a dorsal branch {nt = 



