THE ANCESTRY OP THE OHORDATA. 83 



Lastly, the gill-slits are by their nature repeated structures ; 

 but, seeing that nothing resembling them occurs outside the 

 group,^ their origin and, k fortiori, their repetition has been 

 acquired within it. 



It becomes then probable, from preliminary examination of 

 the morphology of the three typically Chordate features, that 

 their first origin was not in a segmented form. There is also 

 one other structure which certainly points in the direction of 

 an unsegmented animal as the immediate ancestor of the Ver- 

 tebrate. This structure is the liver. Novf, the liver is essen- 

 tially a unique structure in the body which is not repeated. 

 On the Annelid theory of Vertebrate descent it would have to 

 be supposed that the liver either arose as an enlargement of 

 one of the segmental saccules of the gut, or by the coalescence 

 of several. The evidence attainable on this point is distinctly 

 against either of these possibilities ; for the liver of all the 

 Vertebrates, and especially of Amphioxus, is markedly and 

 obviously a single structure, not formed by the coalescence of 

 several, while its asymmetrical position and general appearance 

 favour the view that it is a structure newly formed within 

 the limits of the group, rather than a relic of a paired 

 sacculation. 



Having then disposed of the a priori objections to regard- 

 ing an unsegmented form as a primitive member of the group, 

 the attempt will be made to show that the Enteropneusta 

 occupy this position. After this we will proceed to consider 

 the light which this admission will give on the history of 

 the steps by which the organs of the other Chordata ac- 

 quired their present arrangement, and finally to determine the 

 relation which the various forms included under this head bear 

 to one another. 



The Enteropneusta as Members of the Chordata. 

 The general features of the anatomy of the Enteropneusta 

 place them in a very isolated position. They are extremely 



' Tor Semper's suggestion that the coelomio pores on the heads of some 

 Oligochsets are of the same nature cannot be seriously considered, 



