66 ADAM SEDGWICK. 



of simple ectodermic pits developed for the purpose of aerating 

 those organs, whose position prevented their getting a sufficient 

 supply of oxygen from the external medium or from the water 

 circulating in the alimentary cavity. It must be remembered 

 that there was no vascular system in this ancestor, and that 

 therefore the living protoplasm of all parts of the body had to 

 obtain its oxygen directly from the external medium. This 

 method of aeration has persisted at the present day in certain 

 Medusae (sub-genital pits), in the Tracheate Arthropoda, 

 and has left its trace in the Vertebrata in the canal of the 

 central nervous system. 



On this hypothesis the complicated distribution of trachese 

 receives a physiologically satisfactory phylogenetic explanation. 



The tracheae were at first simple pits of ectoderm 

 in a diploblastic animal, and they gradually became 

 more complicated and branched as the other organs 

 also became more complicated and folded. 



The development of tracheae fits in perfectly well with this 

 view. 



The tracheal respiration is then a primitive method of res- 

 piration, which has persisted in but few of the Triploblastica. 

 It had its origin at a time before the vascular system was 

 developed, and its essence consists in the fact that the living 

 protoplasm takes its oxygen direct from the external medium. 

 On this hypothesis the central canal of the central nervous 

 system was a respiratory organ in a diploblastic Vertebrate 

 ancestor without a well developed vascular system. 



As soon as the vascular system became well developed, and 

 the vascular fluid capable of carrying oxygen, the respiratory 

 organs became localised. A special localisation of tracheae is 

 found in the pulmonary sacs of the Scorpion. In other 

 animals external appendages have arisen. But in Verte- 

 brata, Balanoglossus, and Ascidians, the circulation of 

 water over the surface of the endoderm has been more deve- 

 loped. In the Diploblastic ancestor respiration was, as I have 

 stated, partly efl"ected by water circulating in the alimentary 

 cavity. It entered by one end of the mouth and passed out 



