512 



HENRY M. BKKNARl), 



glands. With this I agree, but propose to specify these dermal glands, 

 and to endeavour to show that they were the bristle or acicular 

 glands of the original Chœtopod. 



This homologisiug of tracheœ with setiparous sacs obviously meets 

 the objections of those who assert that no dermal glands are known 

 in the Annelids which could have given rise to tracheaî, setiparous 

 glands having apparently been overlooked. But since we know that 

 the bristle sacs, even among the Chictopod Annelids themselves, are 

 plastic organs, and may either secrete a bristle or change into long 

 coiled "spinning" glands (e. g. Pohjodontes), we may, I think, fairly 

 ask why they could not form tracheae? 



Fig. 1 is a diagram of an Annelidan bristle, growing from a single 

 cell at the bottom of an ectodermal pit. It 

 seems clear that from the very first air 

 might get down between the bristle and the 

 walls of the pit, especially if there were any 

 irregularities in the smoothness of the former. 

 There seems to be no limit to the modi- 

 fication of bristles for far less essential 

 functions than that now under discussion. 

 Every diminution in the size of the bristle 

 would be an advantage, and its ultimate dis- 

 appearance would leave an ectodermal respi- 

 ratory pit which might well have been the 

 starting point of all knowii forms of tra- 

 cheae. 



That such a pit as this was, as a 

 matter of fact, the starting point of all forms 

 of tracheœ there can, I think, be little doubt. 

 The further development of its proximal end 

 either to ramify through the body to aerate the separate tissues directly, 

 or to form tufts or leaves for the aerating of blood streams presents 

 little difficulty. 



I may perhaps here mention, as no part of the argument, that 

 in certain Insect larvu» the stigmata or openings of the tracheir show 

 an extraordinary resemblance to hair papilhr. No definite argument, 

 of course, can be founded upon this extraordinary similarity. The 

 projection of the end of a chitinous tracheal tube above the level of 

 the cuticle, so that the opening lies on a papilla is a very natural 

 phenomenon. Though no argument can be built on it, it at once be- 



Fig. 1 . Diagram of an Ad- 

 nelidan bristle , growing from 

 the bottom uf an ectodermal 

 pit. (From Boas, Lehrbuch der 

 Zoologie.) 



