278 Proceedings of Royal Society of Edinburgh. [sess. 
and from this would lead four inter-radial sinuses below the nerves. 
On the assumption of bilateral symmetry, as described, two of the 
radial nerves, with part of the ring nerve, would form a pre-oral band, 
the other two between mesocoeles and metacoele would form a post- 
oral band, whilst the rest of the ring-nerve would form ventral cords 
and a post-anal ring. These three nerve-rings are connected 
with the formation of three archimeric ciliated bands which, sooner 
or later, replace the diffuse ciliation of 
earlier stages ; one of these is pre-oral, 
the prototroch (syn. ceplialotroch), and 
two post-oral, the mesotroch (or bran- 
chiotroch) and the metatroch (peri-anal). 
These bands do not persist in present- 
day Ar chi-coelomata , but are found in 
their ontogenetic stages. The main 
ganglion would be moved forward into 
intimate contact with the main animal 
organ, the protocoele, and the sub-neural 
blood-sinus would follow the same 
course. 
The nerves, to a late stage, remain in 
connection with the ectoderm. 
We have thus been enabled briefly 
to sketch the leading characters of an 
ideal ancestor of the Coelomata , as may 
be derived by general principles of 
zontal section through evolution, and the assistance obtained 
hypothetical ancestor of by the acceptance of the theory of the 
the Ar chi- coelomata. . •, ,, , <. i,, 
coelome and that of the blastopore, 
as stated in the commencement. 
Such a type, by the very nature of the case, cannot be found 
living at the present day, for in giving rise to higher types it has 
loug since ceased to be.* 
Fig. 14. — Diagrammatic hori- 
* It will he seen that in nearly all these features Actinotrocha is the em- 
bodiment of this morphological conception of the ancestor from which all the 
Ar chi- coelomata have been derived, and it is remarkable how this larva might 
be, and indeed has been, mistaken for that of nearly all the groups of Archi- 
ccdomata in turn. Cf. Q. J. M. S., vol. xxxviii. p. 282. 
