LARVAL FORMS : THEIR NATURE^ ORIGIN AND AFFINITIES* 399 
dermata, and Actinotrocha (?), implies that these larvse retain, 
SO far, more primitive characters than the Pilidium. The 
same may be said of the alimentary diverticula. There are 
Fig. 18. — T/iree Diagrams representing the ideal Evolution of various Larval 
forms. A. Ideal ancestral larval form. B. Trochosphere larva. 
C. Ecbinoderm larva, m. mouth ; an. anus ; st. stomach ; s.g. supra- 
oesophageal ganglion. The black lines represent the ciliated bands. 
thus indications that in two important points the Echino- 
derm larvse are more primitive than the Pilidium. 
The above conclusions, with reference to the Pilidium and 
Echinoderms, involve some not inconsiderable difficulties, and 
suggest certain points for further discussion. 
In the first place it is to be noted that the above specu- 
lations render it probable that the type of nervous system 
from which that found in the adults of the Echinodermata, 
Platyelminthes, Chsetopoda, Mollusca, &c., is derived, was a 
circumoral ring, like that of Medusse, with which radially 
arranged sense organs may have been connected ; and that in 
the Echinodermata this form of nervous system has heen 
retainedy while in the other types it has been modified by the 
anterior part, having given rise to supra-oesophageal ganglia 
and organs of vision; which were formed owing to the assump- 
tion of a bilateral symmetry, and the consequent necessity for 
the sense organs to be situated at the anterior end of the body. 
If this view is correct, the question arises as to how far the 
posterior part of the nervous system of the Bilateralia can 
be regarded as derived from the primitive radial ring. 
A circumoral nerve-ring, if longitudinally extended, might 
give rise to a pair of nerve-cords united in front and behind — 
exactly such a nervous system, in fact, as is present in 
