454 Causes and Course of Organic Evolution 



nearer to the cyclostomes than to higher nemerteans, it has 

 become diA'erted from the main Hne in structural modification. 



But, as already indicated, while we start with living cyclo- 

 stomes as existing types of Craniata, we regard even these as 

 somewhat lateral derivatives from a set of stem-types that 

 connected the nemerteans with the Gymnophiona (Cseciliada) 

 or a nearly allied group. Amongst fishes, then, the Cyclo- 

 stomata alone ^dll receive our immediate attention. For, as 

 will be explained somewhat fully later, we would regard the 

 scaled fishes or Gnathostomata, i. e., all except the cyclostomes, 

 as being derived A'ery early from a separate ancestry than 

 that which directly originated the cyclostome-csecilian series. 



That the cyclostomes differ very fundamentally from the 

 other groups of fishes is a fact which has often been stated 

 by naturalists. The glandular scaleless skin, the soft circular 

 oral area, the branchial sacs, the absence of lower jaw and of 

 limbs, the numerous segmental areas and the mode of origin 

 of the spinal nerves, the rudimentary character of the excretory 

 system, and the history of development are all diagnostic 

 features. The five great remaining orders of fishes diverge 

 so much from the above, and agree so well amongst them- 

 selves, in broad details, that they and the cyclostomes must 

 early have diverged as two separate phyla. One group of 

 the fishes however, the Dipnoi, includes species which show 

 so many features suggestive of amphibian or even reptilian 

 structure that they have often been regarded as likely ances- 

 tors of the highest vertebrates. When minutely compared, 

 however, the inference does not seem warranted. Rather 

 the resemblances are to be viewed as cases of parallelism or 

 homoplasy in evolutionary history. 



In line with evidence advanced in a previous chapter, the 

 strong probability is that the entire group of cyclostomes 

 as well as protocyclostomes were fresh- water in origin. The 

 almost world-wide distribution of the few living species sug- 

 gests also, apart from structure or affinities, that the group 

 is an ancient one. But, from the softness of all jxirts except 

 the horny teeth, one need not wonder that fossil specimens 



