THE ANCESTRY OF THE OHORDATA. 85 



gill-slits are formed as regular fusions and perforations of the 

 body wall and gut from before backwards. Hence the three 

 features which alone distinguish Chordata from other animals 

 are present, and assocjjited from an early period in develop- 

 ment. Added to this the minor features of Chordate anatomy 

 are also represented by (1) the origin of the mesoblast; (3) 

 the remarkable asymmetry of the anterior parts ; (3) the 

 opercular fold ; (4) the excretory funnels opening into the 

 atrial cavity thus formed. Prom all these facts we may form 

 a preliminary conclusion that the Enteropneusta bear some 

 relation to the Chordata. We will now discuss what relation 

 this isj and before doing so we must determine what relative 

 importance is to be attributed to the two modes of develop- 

 ment known to occur, the one largely embryonic the other 

 pelagic. 



In our present state of ignorance as to the mode of develop- 

 ment of Tornaria and of the details of its later stages, it is 

 difiBcult to compare these two modes, but the question as to 

 which is to be regarded as primitive is probably a part of the 

 larger question as to the comparative likelihood of the pre- 

 servation of ancestral features in the free or in the protected 

 developments. This question cannot be fully gone into here. 

 No general answer has as yet been given to it, and since the 

 balance of probability is very nearly divided between these two 

 possibilities we may be right in assuming either of them to be 

 correct. For the purposes of the following argument it will be 

 assumed that, on the whole, development within an egg-shell, 

 as involving a less complicated struggle with environmental 

 forces, is less subject to variation than that in the open sea, and 

 consequently is more likely to preserve ancestral features. 

 Besides this, in the special case before us, the adult structure 

 is practically conclusive against Echinoderm affinities, to 

 which the pelagic development would point if regarded as 

 primitive. 



Assuming, then, that the development of B. Kowalevskii 

 is more primitive than that involving a Tornaria stage, the 

 following features are of great importance : 



7 



