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 embryological 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? and Van Wijhe? 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 opimion 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. Cambridge Phil. Soc., iv., p. 825; 1883. 
2 Zool. Anz., 1884, p. 683. 
