THE ANCESTRY OF THE VERTEBRATES 529 



There seems thus no doubt that in Amphioxus we have a 

 genuine, although somewhat modified, ancestor of the verte- 

 brate group, the sole survivor of a lost race, the more typical 

 members of which were probably simpler and more like the 

 true vertebrates than is their present-day representative. This 

 conclusion serves, however, only to put the question one stage 

 further back, and we now ask it in this form: What is the 

 relation of Amphioxus to other invertebrates? In order to sum- 

 marize the most fundamental characteristics of Amphioxus, 

 those should be selected which it possesses in common with 

 the vertebrates, since characters possessed by Amphioxus alone 

 may, with great probability, be considered secondarv modifi- 

 cations, and thus unrepresented in their ancestors. 



Thus reduced, search should be made for an animal with a 

 notochord, a dorsal nervous system, and a pharynx perforated 

 by gill-slits, and possessed of a mid-ventral endostyle. Some 

 have included among the essentials a pronounced segmentation, 

 that shows itself in the muscular and nephridial systems, but 

 a study of segmentation in general leads to the opinion that 

 the segmentation of an animal, either expressed in the body- 

 wall, or by the repetition of some of the organs, is not a 

 fundamental character, but one easily acquired, and thus is 

 not to be considered necessarily as an essential characteristic 

 of the group from which Amphioxus and its allies have come. 



The direction of the search thus defined, one is led inevitably 

 to another isolated and problematic group of invertebrates, 

 which have been variously classified among molluscs, worms, 

 and other comprehensive and noncommittal groups, which 

 have had, in short, about the same treatment as that accorded 

 to Amphioxus. This group differs from the latter, however, 

 in that, although isolated, it is extremely rich in the species 

 still extant, and is represented by forms so variously modified, 

 and so widely different from one another, that they form sev- 

 eral Orders and numerous Families, thus showing all possible 

 modifications of their fundamental plan. 



This group is that of the Tunicata, a Class of marine ani- 

 mals, some of which are free-swimming throughout life, while 



