178 ARTHUR WILLEY. 



vessel being dorsal in the former and ventral in the latter.i 

 This fact^ according to Spengel, is in itself almost enough to 

 prove that the gills of the Enteropneusta and of Amphioxus are 

 essentially different structures, and that they do not correspond 

 with one another morphologically. It may, indeed, be said to 

 be SpengeFs strongest point in his objection to the supposed 

 Chordate affinities of the Enteropneusta. But, by applying 

 the above-mentioned principle of correlation to the elucidation 

 of this problem, these and other differences may be viewed 

 from quite another aspect. 



The question, with what other characteristic in the organi- 

 sation and mode of life of the Enteropneusta the dorsal posi- 

 tion of the gills and gill-pores may be correlated, is not con- 

 sidered by Spengel. 



Balanoglossus (employing the name in the wide sense) is a 

 creeping animal,^ and the ventral surface, as in all creeping 

 animals, is the locomotor surface. Some animals may swim 

 on their backs and others on one side, but all who creep do 

 so on their ventral surface. It is inconceivable that gills or 

 gill-pores could occur on the locomotor surface. 



On the contrary, Amphioxus, when active, is essentially a 

 swimmer, and it can no more creep than Balanoglossus can 

 swim. There is, therefore, no such locomotor surface in 

 Amphioxus, and the dorsal region of the primitive alimentary 

 canal is converted into a skeletal support for the body, viz. 

 the notochord. 



The gill-slits and gill-pores of the Enteropneusta are placed 

 dorsally, therefore, in correlation with the locomotor function 

 of the ventral surface, the latter not having such a function in 

 Amphioxus ; and the general homology between the pharyn- 

 geal apparatus in the two types is not thereby prejudiced. 



1 That such a difference in the direction of flow of the blood should not be 

 overrated in the Protochordata is shown by that very well-known faculty of 

 the Tunicate heart of reversing its action and consequently the direction of 

 propulsion. 



-- 2 The kind of burrowing undertaken by Balanoglossus is a variety of 

 creeping, but it creeps too, apart from its burrowing propensities, 



