THE ANCESTRY OF THE CHORDATA. 537 
which, temporarily, is satisfying, and at least provides a point 
of departure for criticism. But in the case of the Chordata 
there are none of these common features. The three characters 
which unite them, the notochord, the gill-slits, and the rela- 
tions of the nervous system, are limiting and exclusive, and 
without parallel in any forms outside the Chordate group. 
So strongly has this fact been felt by many of those morpho- 
logists who have already dealt with the pedigree of the group, 
that they have practically abandoned the attempt to find 
homologies for these features among the Invertebrates; for it 
is impossible to take seriously such suggestions as, for example, 
that the notochord may be compared to, generally, the sacs of 
the Capitellidz, the “ siphons ” of any of various Invertebrates, 
the “ giant-fibres” of Earthworms, or the crystalline style of 
Anodon, Lach of these structures has been in turn suggested, 
together with many others, as offering something with which 
to compare the notochord. In the same way Semper argues 
that the vetebrate gill-slits have an obvious similarity to 
certain pores which he has found in the heads of certain 
Oligochzta (Nais), while other authors see a striking resem- 
blance between them and the Chetopod segmental organ, and 
so on. 
In seeking, then, for the proximate ancestors of Chordata, 
the Chordate features have been disregarded, and another 
character of the vertebrate animal has been selected as 
offering a more probable basis of operations. The character 
which has in this way been chosen as the point of departure is 
that of metameric segmentation. By thus setting aside 
the questions arising out of the notochord, &c., and speculating 
upon the segmentation of the body, the conclusion is soon 
reached that some Annelid was the immediate ancestor sought. 
This view has found its chief exponents in Dohrn and 
Semper, and has been generally supported by Haeckel and by 
most of the popular exponents of evolution. 
It would be unprofitable to recapitulate here the numerous 
morphological difficulties as to the primitive mouth, &c., which 
arise if this theory be received. Many objections of this kind 
