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 

 relationships 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 anatomical 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 complete 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 

 relationship with the Amphibia, and in the mode of formation 

 of the nervous system Petromyzon exhibits a peculiar modi- 

 fication, otherwise only known to occur in Teleostei and 

 Lepidosteus. 



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

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



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

 chapter : 



I. Hyperoartia ex. Petromyzon. 

 II. Hyperotreta ex. Myxine, Bdellostoma. 



2 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 Ur sprung d. Wirbelthiere, etc. Leipzig, 1875. 



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