THE ANCESTRY OF THE CHORDATA. 551 
Lastly, the gill-slits are by their nature repeated structures ; 
but, seeing that nothing resembling them occurs outside the 
group,' their origin and, 4 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. Now, 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 
obyiously 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 odjections to regard- 
ing an unsegmented form as a primitive member of the group, 
the attempt will be made to show that the Eteropneusta 
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 
1 For Semper’s suggestion that the ccelomie pores on the heads of some 
Oligochats are of the same nature cannot be seriously considered. 
