LARVAL FORMS. 385 



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 Ccelenterata. 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 speculation adopted 

 in this section, to prove that its affinities to the Trochosphere, 

 striking as they appear to be, 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 Ccelenterata have certain characters 

 in common, it has \&&\ provisionally assumed that the characters 

 have been inherited from a common ancestor ; and an attempt 

 has been made to determine (i) 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 Ccelenterata may be divided 

 into six groups enumerated on pages 370 to 373. 



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 



15. in. 25 



