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 (gnatho- 

 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 

 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 Hkely 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 from 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 less 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: 



1. 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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