13983) ANNULATE ANCESTRY OF THE VERTEBRATA 27 
standing how the same factors which concentrated the kidneys 
behind the stomach would also assign the same place to the genital 
bodies. In the Arachnida the genital bodies have to accommodate 
themselves to the spaces left among the caeca of the alimentary 
system. 
The body-cavity.—There has hitherto been no satisfactory 
reconciliation of the embryological facts that, while the neural plate 
and notochord, the two most characteristic vertebrate structures, are 
primitively unsegmented, apparently indicating an unsegmented 
ancestral form, the body-cavity appears as a definite series of 
coelomic cavities (e.g. in Amphioxus) apparently indicating equally 
emphatically that the ancestral form was segmented. <A confusion 
of types, if ever there was one! Now I make bold to suggest that 
the assumed modifications here sketched supply us with a possible 
solution. The series of archenteric cavities which, in our annulate 
ancestors, encircled the body below the skin, were gradually pre- 
vented, in the early vertebrates, from invading the dorsal region, 
because that region in becoming secondarily specialised into a 
neuro-muscular region, essential to the free life of the larva, 
was developed as early as possible in the ontogeny. Hence, in the 
embryo, the metamerism is confined to the vegetative region; its 
invasion of the dorsal region is secondary, in that a gradual impress 
is made upon the originally unsegmented notochord and on the 
points of departure of the spinal nerves by the segmented muscles 
and developing skeletal rings. 
The moving forward of the anus and the coiling of the 
alimentary canal—tThe periodic distension of the stomach, by 
stretching the walls of the body, would tend to form a space into 
which the rest of the alimentary canal would gradually withdraw, 
simply from following the direction of least pressure. This process 
might be furthered by the additional advantage to the animal of a 
tail which could be used as a purely locomotory organ unhampered 
by any other organs. For it is obvious, on the one hand, that a 
rectum periodically charged with more or less solid faeces, the 
residue of the solid matter swallowed, would seriously impede the 
free movements of such a tail, while, on the other hand, the dis- 
tension of the middle body, hindering the original serpentine motion 
by which our annulate ancestors progressed through the water, 
would lead to the necessity of gradually specialising the tail as the 
chief organ of locomotion. The forward movement of the anus and 
the coiling of the intestine would therefore be natural results of 
this new differentiation of the body into a swollen anterior portion 
and a flexible tail (cf the developing tadpole). 
Before concluding, I wish again to emphasise the drift of this 
article, which is simply to suggest a possible method by which the 
