402 
F. M. BALFOUR. 
system of the adult, points to the conclusion that the adult 
Echinodermata hme retained^ and not, as is now usually 
held, secondarily acquired, their radial symmetry ; and if 
this is admitted it follows that the obvious bilateral sym- 
metry of Echinoderm larvae is a secondary character. 
The bilateral symmetry of many Coelenterate larvae (the 
larva of ^ginopsis, of many Acraspeda, of Actinia, &c.), 
coupled with the fact that a bilateral symmetry is obviously 
advantageous to a free-swimming form, are quite sufficient to 
show that this supposition is by no means extravagant; while 
the presence of only tw^o alimentary diverticula in Echino- 
derm larva is quite in accord with the presence of a single 
pair of perigastric chambers in the early larva of Actinia, 
though it must be admitted that the derivation of the water- 
vascular system from the left diverticulum is not easy to 
understand on this view. 
Fig. 20. — Two Stages in the development oj Mitraria. (After Metsclini- 
koff.) m. mouth ; an. anus ; sg. supra-oesophageal ganglion ; br. and 
b. provisional bristles ; pr.b. prseoral ciliated band. 
A difficulty in the above speculation is presented by the 
fact of the anus of the Echinodermata being the permanent 
blastopore, and arising prior to the mouth. If this fact has 
any special significance, it becomes difficult to regard the larva 
of Echinoderms and that of the other types as in any w^ay 
related ; but if the view's already urged, in a previous essay on 
the germinal layers, as to the unimportance of the blastopore, 
are admitted, the fact of the anus coinciding with the blasto- 
