THE ANCESTRY OF THE OHOEDATA. 69 



which, temporarily, is satisfying, aud 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 Capitellidse, the " siphons " of any of various Invertebrate.s 

 the " giant-fibres " of Earthworms, or the crystalline style of 

 Anodon. Each 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 

 Oligochseta (Nais), while other authors see a striking resem- 

 blance between them and the Chaetopod 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 



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