24 NATURAL SCIENCE [July 



sudden contractions of the dorsal muscles. Its subsequent develop- 

 ment, as it became invested with cartilaginous and bony rings, is 

 already within the vertebrate domain. 



The spinal cord. — Great muscular development is impossible 

 without corresponding nerve-development. Hence it seems to me, 

 if the division of labour here assumed is admitted, a nerve-strand 

 would develop down the middle between the muscular bands, not 

 necessarily as an altogether new structure but as a new condensation 

 of elements probably already present. I have always hitherto been 

 of the opinion that the enibryological nerve-plate, which subsequently 

 forms the well-known groove and neurenteric canal, was the remains 

 of larval adaptations ; but from the point of view now suggested it 

 appears that the process might actually represent, in a very abbre- 

 viated form, the gathering together of the originally scattered nerves 

 which supplied the dorsal muscular region into a central strand for 

 more perfect co-ordination. Perhaps, also, since respiration in the 

 Hirudinea is effected solely by the skin, the neurenteric canal may 

 have been a temporary arrangement for aerating this important 

 nerve area as Sedgwick 1 and Van Wijhe 2 suggested long ago. 

 Be this as it may, the appearance of the spinal cord itself, 

 possibly as a new development of pre-existing elements, could be 

 considered as a natural consequence of the division of labour above 

 postulated. With regard to the great development of the anterior 

 portion of the spinal cord, the brain, we should not be far wrong if 

 we referred this to a great improvement in the organs of sense 

 required by the new race of swift, rapacious carnivores. 



Concurrently with the development of this new nerve- cord, the 

 primitive ventral nerve-cord of the annulate would be slowly de- 

 generating, not only because of the transference of the chief muscular 

 activity to the dorsal region, but because the ganglionic chain itself 

 would be positively incapacitated from fulfilling its functions 

 not only by the periodical distension but by the more constant 

 pressure of the alimentary canal weighted with solid food. In 

 this comparatively simple manner, then, I suggest that the prob- 

 lem of the nerve-cords might be solved, and one of the difficulties 

 in the way of the annulate ancestry of the vertebrates be avoided. 

 Of course, it must remain a matter of opinion whether the 

 assumptions here made are really preferable to the (to my mind) 

 desperate hypothesis otherwise difficult to avoid, that our worm 

 ancestor turned over on to its back, that its ventral cord became our 

 dorsal cord, and that its old mouth vanished and a new one 

 developed. 



Concurrently with these specialisations of the dorsal neuro- 



1 Proc. Ga?nbridge Phil. Soc., iv., p. 32~> ; 1883. 



2 Zool. Anz., 1884, p. 683. 



