398 
F. M. BALFOUR. 
Before proceeding to further considerations as to the origin 
of the Bilateralia, suggested by the Pilidium type of larva, 
it is necessary to enter into a more detailed comparison be- 
tween our larval forms. 
A very superficial consideration of the characters of these 
forms brings to light two important features in which they 
differ, viz. : 
(1) In the presence or absence of sense organs on the 
prgeoral lobe. 
(2) In the presence or absence of outgrowths from the 
alimentary tract to form the body-cavity. 
The larvse of the Echinodermata and (?) Actinotrocha are 
without sense organs in the praeoral lobe, while the other 
types of larvse are provided with them. Alimentary diverticula 
are characteristic of the larvse of the Echinodermata and of 
Tornaria. 
If the conclusion already arrived at, to the effect that the 
prototype of the six larval groups was descended from a 
radiate ancestor, is correct, it appears to follow that the 
nervous system, in so far as it was differentiated, had primi- 
tively a radiate form, and it is also probably true that there 
were alimentary diverticula in the form of radial canals, (wo 
of which may have given origin to the paired diverticula 
which become the body-cavity in such types as the Echino- 
dermata. If these two points are granted, the further con- 
clusions seem to follow — (1) that the ganglion and sense 
organs of the preeoral lobe were secondary structures which 
arose (perhaps as differentiations of the original circular 
nerve-ring) after the assumption of a bilateral form ; and [2) 
that the absence of these organs in the larva? of the Echino- 
