of the integument or outer surface of the body, and their gills are
merely expansions of this. In the annelid they are projections of
the parapodia, in the mollusk expansions of the skin, where the foot
or creeping sole joins the body. Why did the vertebrate take a new
and strange, and, at first sight, disadvantageous mode of
breathing? There must have been some good reason for this. The most
natural explanation would seem to be that he had no projections on
his outer surface which could develop into gills, and farther, that
he could not afford to have any. Now projections on the lower
portion of the sides of the body would be an advantage in creeping,
but a hindrance in any such mode of swimming as we have described,
or indeed in any mode of writhing through the water.

Furthermore, if he lived, not a creeping life on the bottom, but
swimming in the water above, he would have to live almost entirely
on microscopic animals and embryos; and these would be most easily
captured by a current of water brought in at the mouth. The whole
branchial apparatus in its simplest forms would seem to be an
apparatus for sifting out the microscopic particles of food and only
later a purely respiratory apparatus. Moreover, we have seen that
the parapodia of annelids naturally point to the development of an
external skeleton, for their muscles are already a part of the
external body-wall and attached to the already existing horny
cuticle. The logical goal of their development was the insect.

Now I do not wish to conceal from you that many good zoölogists
believe that the vertebrate is descended from annelids; but for this
and other reasons such a descent appears to me very improbable. It
would seem far more natural to derive the vertebrate from some free
swimming form like the schematic worm, whose largest nerve-cord lay