CYCLOSTOMES. 219 



In dealing with the classification of the vertebrates many dif- 

 ferent ideas exist, not only with regard to the interrelationships 

 of the various groups, but with their co-ordination as well. This 

 is due to several causes. Among them may be mentioned the 

 fact that only in exceptional cases among the fossil vertebrates 

 are other structures than the skeleton preserved, and for this 

 reason our classifications have of necessity been too largely 

 based upon osteological characters. Again, there is a great dif- 

 ference in the numbers of species in the different groups ; thus 

 the cyclostomes, one of the two great divisions of vertebrates, 

 contain less than a score of species, while of birds about 

 twelve thousand ' species ' are known. As a result, the group 

 of aves has been subdivided to an extent unknown in other 

 classes. Divisions which elsewhere would be regarded as fami- 

 lies are here raised to ordinal rank, and other subdivisions cor- 

 respondingly magnified. In the following pages it has been 

 the attempt to preserve a proper co-ordination of groups, — to 

 maintain a classificatory perspective. 



The vertebrate phylum may be divided into cyclostomes and 

 gnathostomes. 



Series I. Cyclostomata (Agnatha). 



Eel-like vertebrates without paired appendages ; mouth suc- 

 torial, jaws lacking ; olfactory organ single and median ; optic 

 nerves going to the eyes of the same side; gills 6-14 pairs in 

 saccular pouches. The cyclostomes include but a single class, 

 — the Marsipobranchii. 



Class I. Marsipobranchii (Myzontes). 



The marsipobranchs are undoubtedly the lowest vertebrates ; 

 but there is yet a question as to how far their simple structure 

 is the result of a primitive condition, and how far it has been 

 caused by degeneration. The body is eel-like ; and all traces of 

 paired fins are absent, unless, as Dohrn suggests, two slight folds 

 near the vent are the remnants of ventral fins. A median fin 

 occurs, which may be continuous, or may be differentiated into 



