LARVAL FORMS. 317 



completely retained their radial symmetry. How far Actinotrocba' is 

 related to the Echinoderm larvae cannot be settled. Its characters 

 may possibly be secondar}^ like those of the mesotrochal larvae of 

 Chaetopods, or they may be due to its having branched off very 

 early from the stock common to the whole of the forms above the 

 Coelenterata. The position of Tornaria is still more obscure. It is 

 difficult, in the face of the peculiar water-vascular vesicle with a dorsal 

 pore, to avoid the conclusion that it has some affinities with the 

 Echinoderm larvae. Such affinities would seem, on the lines of specu- 

 lation adopted in this section, to prove that its affinities to the Trocho- 

 sphere, striking as they appear to be, are secondary and adaptive. 

 From this conclusion, if justified, it would follow that the Echinoder- 

 mata and Enteropneusta have a remote aucestor in common, but not 

 that the two groups are in any other way related. 



General conclusions and summary. Starting from the demon- 

 strated fact that the larval forms of a number of widely separated 

 types above the Coelenterata have certain characters in common, it 

 has been provisionally assumed that the characters have been inherited 

 from a common ancestor ; and an attempt has been made to determine 

 (1) the characters of the prototype of all these larvae, and (2) the 

 mutual relations of the larval forms in question. This attempt started 

 with certain more or less plausible suggestions, 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 305 to 307. 



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 surface. 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 growing 

 out to form 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-cesophageal ganglia, and 

 the optic organs connected with them ; while the posterior part of 

 the nerve-ring formed (?) the ventral nerve-cords. The body cavity 

 was developed from two of the primitive alimentary diverticula. 



The usual view that radiate forms have become bilateral by the 

 elongation of the aboral dome into the trunk is probably erroneous. 



4. Pilidium is the larval form which most nearly reproduces 



' It is quite possible that Phoronis is in no way related to the other Gephyrea. 



