VERTEBRATA. 
309 
from the middle of the ear to the base of the nose, and 
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. 168 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 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 ), as in some Reptiles 
and Fishes. 
There are two great divisions of the subkingdom, 
Acrania and Craniota , or Vertebrates without skulls and 
those with skulls. 
The Craniota are divided into five great classes: Fishes , 
Amphibians , Reptiles , 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 
bl ood-corp uscles (Ichthyopsida). 
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 
