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, a¢., 1n 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 nay 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- 
