CHAPTER IV 

 CLASS I. CYCLOSTOMATA 



The name " round-mouth eels" is often applied to these fish -like 

 forms to distinguish them from the "jaw-mouth" fishes 



tomes). The group consists of two distinct types, the " hag-fishes" 

 and the lampreys. Both of these have a superficial resemblance-to the 

 eels, but they differ from the group to which the eels belong (Pisces) / 

 in many important respects. Some of the differences are to be V 

 interpreted as evidences of primitiveness and others of specialization 

 and degeneration. Since there are no certain fossil remains of the cy- 

 clostomes, it is an exceedingly difficult matter to decide as to which 

 of their characters are palingenetic (truly primitive) and which are 

 csenogenetic (due to specialization or degeneration) . Evidently, how- 

 ever, the characters that are of universal occurrence in the group are 

 more likely to be primitive than are those in which the "hags" dif- 

 fer from the lampreys. 



The cyclostomes are a minor class of vertebrates as compared with 

 the other five classes, since there are only a few genera and species. 

 They are also of secondary significance phylogenetically; for they are 

 believed to represent a comparatively unsuccessful lateral branch of 

 the vertebrate ancestral tree, that came off probably from an Amphi- 

 oxus-like stock prior to the origin of the fishes proper. If this view is 

 valid, the true fishes and all of the other vertebrate classes repre- 

 sent an evolutionary series totally independent fron^the cyclostomes. 

 Any attempts, therefore, to establish detailed homologies between the 

 two groups must be viewed with suspicion. On the whole, the cyclo- , 

 stomes have departed Jess .widely from the Amphioxus-like prototype ** 

 than have the fishes, and in that sense they represent a lower grade 

 of vertebrate organization. 



The characters in which the cyclostomes in general differ from the 

 fishes are as follows: 



External features: 



A) No jaws. Attempts have been made to homologize the so-called 

 "tongue cartilages" with the first visceral or mandibular arch, 

 but the comparison is far-fetched. 



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