470 Causes and Course of Organic Evolution 



From the foregoing comparative studies we are compelled 

 to draw the conclusion that the cyclostomes are not only in 

 almost direct line with nemerteans, they are equally removed 

 from the true fishes. Further they show so many graded 

 morphological connections with the Apoda, and these again 

 with many living Urodela, that all of them, or some not dis- 

 tantly removed ancestral types, form a common and graded 

 line of ascent of surprising completeness. 



But, further, amongst the Urodela a progressive relation- 

 ship in the li^^ng types seems fairly well traceable, while evi- 

 dence for any extensive degeneracy of genera or families seems 

 practically negligible. For, while a certain morphological 

 overlapping or crossing of genera and families is at times ob- 

 servable, most structural details point to the Sirenidse as the 

 most ancient and primitive structurally of those now living, 

 and as the group that most nearly approaches to the Apoda. 

 The taxonomic and phylogenetic connections even seem in 

 a general way to be indicated by the number of vertebrae in 

 the entire vertebral column. Thus in Apoda 200 to 300 may 

 be counted, in Siren 90-100, in Amphiuma 100-150, in Proteus 

 60-70, in Cryptobranchus 50-60, in Amhlystoma 45-50, in Sala- 

 mandra 45-50, in Triton 40-50, in Spelerpes 35-40. The num- 

 bers unquestionably vary — at times considerably — in different 

 species or genera, even in different indi\dduals as to tail ver- 

 tebrae especially, but the average seems to agree with other 

 characters that may now be quickly re\dewed. 



In the Sirenidse the gills are perennibranchiate ; hind limbs 

 are absent; the fore limbs have four digits; the eyes are devoid 

 of eyelids; the jaws are toothless, but horny teeth and horny 

 jaws may occur in larvae (138: 243); the skeleton is largely 

 cartilaginous and the vertebrae are pseudocentrous; there are 

 no supra- or basi-occipitals, no intercalary, maxillary, vomero- 

 palatine, prefrontal, or pterygoid bones; there are four epi- 

 branchial cartilages and of the four larval gill clefts the 2nd, 

 3rd, and 4th remain open; the carpal cartilages are few; the 

 alimentary canal is simple. These are all characters that 

 are absent in cyclostomes, because of their greatly lower posi- 



