1898] ANNUL A TE < t Nt 'AW 772 ) r OF THE I 'El! TE1UL ITA 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 Ampkioxus) 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. — The 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 amis 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 



