ELEMENTS OF VERTEBRATE PHYLOGENY 135 



The lowest of the vertebrates are the cyclostomes, so named for their 

 round, suctorial, jawless mouths. The cyclostomes include the hags, 

 whose eyes are microscopic and functionless, and the lampreys (Fig. 60). 

 They are eel-shaped, blood-sucking parasites upon fishes. Some small 



[Higher P l acentals| 

 Insect I vores] 



[Marsupials^ 



[Monotremes 



t Therapsidans 



t [Stegocephalians| !T |Coecilians[ 



iHolosteans', ' ' ^ |Cladi stians[ /^; ^rossopterygiansj 



>^ ^/" 'Dipnoansj 



[Modern Chondrosteons^ 



Primitive 

 Chondrosteans 



L^ 



.Selachians 



Chinnaeras 



I Primitive 

 Elasmobranchs 



Primitive 

 Cyclostomesl 



^Lampreys! 

 ^Hagfishes] 



Fig. 60 — Inter-relations of the major groups of vertebrates. 

 Only those extinct groups (marked f) are shown which actually link up living assemblages. 



freshwater lampreys have given up parasitism and do all of their feeding 

 as larvae, breeding for the first and only time a few months after trans- 

 forming to the adult condition. Parasitic lampreys also breed but once 

 after years of vegetative activity, and then die. Cyclostomes have no 

 scales or paired fins, and many other things about their anatomy are 



