406 
F. M. BALFOUR. 
tion adopted in this essay, to prove that its affinities to 
the Trochosphere, striking as they are, are secondary and 
adaptive. From this conclusion, if justified, it would follow 
that the Echinodermata and Enteropneusta have a remote 
ancestor in common, but not that the two groups are in any 
other way related. 
General conclusions and summary , — Starting from the 
demonstrated fact that the larval forms of a number of widely 
separated types above the Coelenterata have certain characters 
in common, an attempt has been made(l) to determine the 
characters of the common prototype of all these larvae, (^) and, 
the mutual relations of the larval forms in question. This 
attempt started wfith certain more or less plausible sugges- 
tions, the truth of which can only be tested by the coherence 
of the results which follow from them, and their capacity to 
explain all the facts. 
The results arrived at may be summarised as follows : 
1. The larval forms above the Coelenterata may be divided 
into six groups enumerated on pages 391 — 394. 
2. The prototype of all these groups was an organism 
something like a Medusa, with a radial symmetry. The 
mouth was placed in the centre of a flattened ventral sur- 
face. The aboral surface was dome-»shaped. Round the edge 
of the oral surface was a ciliated ring, and probably a nervous 
ring provided with sense organs. The alimentary canal was 
prolonged into two or more diverticula, and there was no 
anus. 
3. The ^bilaterally symmetrical types were derived from 
this larval form by the larva becoming oval, and the region 
in front of the mouth forming a praeoral lobe, and that 
behind the mouth the trunk. The aboral dome became 
the dorsal surface. 
On the establishment of a bilateral symmetry the anterior 
part of the nervous ring gave rise to the supra*oesophageal 
ganglia, and the optic organs connected with them. The 
body-cavity was developed from two of the primitive alimen- 
tary diverticula. 
The usual view that radiate forms have become bilateral 
by the elongation of the aboral dome into the trunk is pro- 
bably mistaken. 
4. Pilidium is the larval form which most nearly repro- 
duces the characters of the larval prototype in the course of 
its conversion into a bilateral form. 
5. The Trochosphere is a completely differentiated bilateral 
form, in which an anus has become developed. The praeoral 
ciliated ring of the Trochosphere is probably derived from 
