308 COMPARATIVE ZOOLOGY. 



another from the forehead to the most prominent part of 

 the upper jaw, will include what is called the facial an- 

 gle, which roughly gives the relation between the two re- 

 gions, and therefore the rank of the animal. 166 In the 

 cold-blooded Vertebrates the brains do not fill the cranium ; 

 while in Birds and Mammals a cast of the cranial cavity 

 well exhibits the general features of the cerebral surface. 167 



All higher Vertebrates are single and free. Mammals 

 bring forth their young alive, having directly nourished 

 them from the mother before birth (viviparous). In almost 

 all the others the nourishment is laid up in the egg, which is 

 laid before hatching (oviparous), or is retained in the mother 

 until hatched (ovoviviparous),&$\\\ some Reptiles and Fishes. 



Of the subkingdom Vertebrata or Chordata there are 

 three great divisions, Urochordata, Acrania, and Craniota. 

 The first division includes the Tunicates, and the second 

 the Vertebrates without skulls e. g., the AmpJiioxus.* 



The Craniota are divided into five great classes : Fishes, 

 Amphibians, Eeptiles, Birds, and Mammals. The first 

 three are "cold-blooded," the other two are "warm- 

 blooded." Fishes and Amphibians have gills during the 

 whole or a part of their lives, while the rest never have gills. 

 Fishes and Amphibians in embryo have neither amnion 

 nor allantois, while the other three are provided with both. 



There are three provinces of skull-bearing Vertebrates. 



Fishes and Amphibians agree in having gills, in want- 

 ing amnion and allantois, and in possessing nucleated red 

 blood-corpuscles (Ichthyopsidd). 



Birds and Reptiles agree in having no gills, but both 

 amnion and allantois, in the articulation of the skull with 

 the spine by a single condyle, in the development of the 

 skin into feathers or scales, and in circulating oval, nucle- 

 ated, red corpuscles (Sauropsida). 



Mammals differ from Birds and Reptiles in having two 

 occipital condyles, and their red blood-corpuscles are not 

 nucleated 168 (Mammalia). 



