PHYLOGENY. 93 



nus represents the class Acrania, which is the ancestor 

 of craniate Vertebrata. 



The phylogeny of the Vertebrata may be repre- 

 sented diagrammatically as follows : 



Aves 



Teliostomata (other! 



TeteStomata (Rhipidopterygia) 



Elasmobr. (Selachii) 

 Hlasmo >ranchii Acanthodii 



Agi atha 

 Tunicata Acrania Enteropneusta 



The Vertebrata exhibit the most unmistakable gra- 

 dation in the characters of the circulatory system. It 

 has long been the custom to define the classes by means 

 of these characters, taken in connection with those of 

 the skeleton. Commencing in the Leptocardii with 

 the simple tube, we have two chambers in the Marsi- 

 pobranchii and fishes ; three in the Batrachia and Rep- 

 tilia ; and four in the Aves and Mammalia. The aorta- 

 roots commence as numerous pairs of branchial arter- 

 ies in the Leptocardii ; we see seven in the Marsipo- 

 branchi, five in the fishes (with number reduced in 

 some) ; four and three in Batrachia, where they gen- 

 erally cease to perform branchial functions ; two and 

 one on each side in Reptilia ; the right-hand one in 

 birds, and the left-hand one in Mammalia. This order 

 is clearly an ascending one throughout. It consists 

 of, first, a transition from adaptation to an aquatic, to 



