CHAPTER V. 



CYCLOSTOMATA 1 . 



PETROMYZON is the only type of this degenerated but primitive 

 group of Fishes the development of which has been as yet studied 2 . 



The development does not however throw any light on the rela- 

 tionships of the group. The similarity of the mouth and other parts 

 of Petromyzon to those of the Tadpole probably indicates that 

 there existed a common ancestral form for the Cyclostomata and 

 Amphibia. Embryology does not however add anything to the anato- 

 mical evidence on this subject. The fact of the segmentation being 

 complete was at one time supposed to indicate an affinity between 

 the two groups; but the discovery that the segmentation is also com- 

 plete in the Ganoids deprives this feature in the development of any 

 special weight. In the formation of the layers and in most other 

 developmental characters there is nothing to imply a special relation- 

 ship with the Amphibia, and in the mode of formation of the nervous 

 system Petromyzon exhibits a peculiar modification, otherwise only 

 known to occ-ur in Teleostei and Lepidosteus. 



Dohrn 3 was the firsb to bring into prominence the degenerate character 

 of the Cyclostomata. I cannot however assent to his view that they are 

 descended from a relatively highly-organized type of Fish. It appears to 

 me almost certain that they belong to a group of fishes in which a true 

 skeleton of branchial bars had not become developed, the branchial skele- 



1 The following classification of the Cyclostomata is employed in the present 

 chapter : 



! Hyperoartia ex. Petromyzon. 

 II. Hyperotreta ex. Myxine, Bdellostoma. 



! The present chapter is in the main founded upon observations which I was able to 

 make in the spring of 1880 upon the development of Petromyzon Planeri. Mr Scott 

 very kindly looked over my proof-sheets and made a number of valuable suggestions, 

 and also sent me an early copy of his preliminary note (No. 87), which I have been 

 able to make use of in correcting my proof-sheets. 



3 Der Ursprung d. 11'irbelthiere, etc. Leipzig, 1875. 



