51 



Recent researches* on the structure and embryology of 

 Peripatus (Onychophora in table) have shown that while it 

 is distinctly one of the Tracheata, in as much as it possesses 

 respiratory organs in the form of tracheae, it still exhibits 

 the Annelidan feature of paired nephridia corresponding to 

 the somites. Peripatus from its geographical distribution 

 must be regarded as a very ancient type, and in all prob- 

 ability it represents the ancestral Tracheata close to the point 

 of their divergence from the Vermes. It has probably 

 become slightly modified by degeneration, but still is 

 extremely useful in helping us to form an idea of the series 

 of changes by which the Tracheata were evolved. The 

 ancestral worm from which the branch started must be 

 regarded as having attained about the same level of high 

 organisation as the form from which the series of Crustacea 

 arose, consequently the two great Arthropodan branches may 

 have diverged from a common ancestor in the Vermes. 



The primitive Tracheata had an elongated worm-like 

 body, which was divided into segments, probably with com- 

 paratively little heteronomy. Each segment was prolonged 

 laterally into a pair of processes, the appendages, which were 

 beginning to be transversely segmented or jointed. The 

 muscular system was becoming more highly differentiated, 

 and the cuticle had partially hardened to form an exoskeleton. 

 The characteristic excretory organs of the Vermes were still 

 retained in the form of laterally placed nephridia, a pair in 

 each segment ; but the typical respiratory system of the 

 Tracheata had commenced to develop, probably at first as a 

 series of slight ectodermal depressions which gradually 

 worked their way deeper and deeper into the tissues until 

 they formed a system of tubes, branching through the body 



* Moseley on the Structure and Development of Peripatus capensis, 

 Phil. Trans., v. 164 (1874) ; and Balfour, Quart. Jour. Micros. Sc, vol. xix 

 (1879) and vol. xxiii, no. xc, p. 213 (1883). 



