64 



KINSHIPS OF CYCLOSTOMES 



of the group. Among its primitive features are to be in- 

 cluded : skeleton and muscles, continuous vertical fin, gill 

 characters (p. 260), viscera (p. 263), urino-genital organs 

 (pp. 266, 270), nervous and circulatory systems (pp. 260, 

 269, and 274). With these must be taken into account: 

 absence of mandible* and of paired fins and girdles; and in 

 addition the remarkable conditions of metamerism (p. 14). 



Little more that a vague kinship between lampreys and 

 fishes has been established by the study of living forms. 

 And, on the other hand, it would appear equally impracti- 

 cable to obtain evidence bearing upon this problem from 

 the side of palaeontology. All that is known of the recent 

 Cyclostomes more than suggests that their soft body struct- 

 ures would prove most unfavourable to fossilization. It 

 would be only, therefore, in the event of some of their 

 ancient members possessing calcified structures that palae- 

 ontology would be able to offer a clue as to their ancient 

 affinities. 



Upon the problem of their descent the evolution of 

 fishes has, however, an undoubted bearing, in suggesting 

 the lines and effects of aquatic evolution and the perma- 

 nence of generalized types. It certainly tells of the ex- 

 treme slowness of the evolution of aquatic forms and con- 

 vinces us that the ancestral Cyclostome could only have 

 occurred in a time stratum exceedingly remote. Palaeon- 

 tology cannot perhaps hope to obtain more than sugges- 

 tions of the ancestral forms, although these, from their 

 generalized characters, may well have survived during geo- 



* The cartilages of the mouth region of Cyclostomes have been homologized 

 with the structures of gnathostomes ; Pollard recently {Anat. Anz. ix, pp. 

 349-359) ascribes a cirrhostomial origin to the mouth parts of a Teleostome 

 (catfish), which the writer cannot believe has been demonstrated; variations 

 in the number, shape, and function of the cartilages of the mouth rim of 

 Cyclostomes might well have occurred within the limits of this ancient group. 



