THE ANCESTRY OF THE VERTEBRATES 519 



forms they are confined to the anterior end. Gills of this sort 

 are well adapted to slow-moving or crawling forms, but when 

 there is a necessity for the development of rapid motion, as is 

 indicated for the direct ancestor of the rapidly moving fishes, 

 such gills, especially if long and fringed, would tend to retard 

 the motion. It would thus be natural to consider that they 

 might wander within the openings of the nephridia, which in 

 annelids lie close to these external gills, and this relationship 

 gives, in its turn, the motive for the secondary connection of 

 such nephridia with the alimentary canal, in order to supply 

 the gills with a current of water. 



The increase in the size of the gills would tend to develop 

 some firmer tissue at their base to support them, and in this way 

 there may have been developed a series of cartilaginous arches, 

 which, together with the gills themselves, may have been at 

 first and for a long time coextensive with the body itself, or 

 have extended at least as far as the anus. When at a later 

 period the gills became restricted to a few anterior pairs while 

 the rest atrophied, the arches accompanying the former would 

 be the persistent gill-arches, and form the visceral skeleton 

 of vertebrates, while the remainder, freed from their gills, 

 and repeating themselves metamerically, would become the ribs. 

 It is even permissible to conceive of the limb skeletons as 

 further derivatives of the metameric system of gill-arches ; per- 

 haps also the original elements of the primordial skull, the 

 trabeculse and parachordals, may be traced to the same source, 

 thus accounting for all parts of the skeleton save the dermal 

 bones, which are integumental, and the notochord, which has 

 already been accounted for. 



Convincing as these comparisons seem when taken by them- 

 selves, the influence of later investigation has tended rather 

 away from the annelid hypothesis, and at present, although 

 there are many investigators who seek the ancestor of verte- 

 brates in some worm-like form, there are few who wish to defi- 

 nitely assert that this ancestor was an annelid. 



The annelid theory rests largely upon the definite body seg- 

 mentation of both these animals and vertebrates, yet segmenta- 



