ANIMAL AFFINITIES 287 



fundamental ones. Thus there are degrees of generality of develop- 

 mental phase. 



All chordate animals whatever have, in their early embryogeny, 

 an axial, stiffening rod that arises as an endodermal invagination. 

 In the cyclostomes this notochord remains throughout life as the 

 functional skeleton. In the Fishes the notochord acquires annular, 

 calcareous rings ; then it becomes replaced by bony segments, 

 or vertebrae, and in the most specialized vertebrates the vertebrae 

 become very complex. 



All vertebrate animals have a propulsive circulatory apparatus 

 w^hich consists of 2 muscular dilatations of the main blood-vessel. 

 In the Fishes the dilatations become the muscular heart consisting 

 of auricle and ventricle. In the amphibia the originally single 

 auricle becomes divided into right and left auricles and then, 

 exceptionally in the Reptiles and universally in the Birds and 

 Mammals, the ventricle also becomes divided. Therefore there 

 is the series : 



2-chambered — >■ 3 -chambered — >■ exceptionally — >- universally 

 heart heart 4-chambered 4-chambered 



heart heart 



(Fishes) (Amphibia) (Some (Birds and 



Reptiles) Mammals) 



The generalized tectonic features in these examples are 

 the notochord and the doubly dilated, muscular, circulatory 

 apparatus — successive modifications elaborate these structures. 

 Thus there are degrees of generality of structural^ or tectonic 

 characters. 



It is this conception of degrees of generality of character that 

 receives expression in rational classifications. 



95^. The Classification of the Chordata. The main 

 divisions are so constituted : 



[Acrania . . Notochord present N 



Chordata. Nutritive organ, the endostyle present E 



' Craniata . . as above N(E) 



also a cranium present C 



and a dorsal nervous system . . . Dn 

 (Cyclostomata . . Craniata as above . . N(E)C Dn 

 p ' . ] Mouth is suctorial 



I Branchiae (gills) are present . . Br 



\Gnathostomata . Craniata as above . . N(E)C Dn 



Branchiae present Br 



Jaws in the mouth J 



