26 NATURAL SCIENCE [July 



might supply us with the possible conditions out of which the 

 breathing organs of the early vertebrates could have developed. 

 While the stretching of the ventral and lateral skin by the dis- 

 tended stomach would hinder circulation in the areas thus affected, 

 the pulsing of the increasingly powerful heart would distend the 

 cutaneous vessels nearest to it, i.e., in the neck region. Hence a 

 possible origin of the external gills. At the same time, the rapid 

 swimming through the water with the mouth frequently, if not 

 always, open for the capture of prey, would keep an almost constant 

 supply of fresh water in the pharynx. Here, also, we should, on 

 account of its proximity to the heart, expect a subsidiary respiratory 

 surface to develop. The actual processes which would lead to the 

 union of the inner and outer respiratory surfaces by means of gill- 

 clefts, it is not easy now even to conjecture. The best suggestion 

 which occurs to me is, that they may have been due to the well- 

 known principle of the increase of respiratory surface by plication, 

 which would inevitably bring the portions of the inner and outer 

 surfaces into increasingly close proximity. We can, however, readily 

 estimate some of the advantages of this development. The clefts 

 would insure a fresh stream of water through the pharynx, this 

 stream would aerate the posterior of the external gills, which, in 

 rapid swimming would be folded back against the body, and thus be 

 screened from the necessary contact with the medium by the anterior 

 gills. Lastly, the clefts afforded retreats into which all the external 

 gills ultimately withdrew, their presence on the exterior being a 

 hindrance to locomotion, and a source of danger in the event of 

 attack. 



Excretion. — In the more primitive of the hirudineans, the 

 segmental organs (nephridia) are arranged laterally along each 

 side of the under surface of the body. It would of course be 

 imperative to protect these from all injurious pressure from the 

 distended alimentary canal. In order to escape this, their relative- 

 position might have been changed, and changed in the following 

 way. The downward distension of the alimentary canal might be 

 expected to force the two rows of primitive nephridia apart until, 

 instead of lying laterally below the alimentary canal, they would 

 come to lie laterally above that organ. This, we might suppose, 

 would be the first change of position, brought about perhaps 

 mechanically and also perhaps partly physiologically, inasmuch as 

 the downwardly pressing intestine would permit a slightly freer 

 circulation above it than below it. A further change would take 

 place when the organs became concentrated behind the stomach ; 

 again, no doubt, in order still further to escape from pressure from 

 that organ. 



The reproductive bodies. — There is no difficulty in under- 



