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. 156 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. 157 
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 ), as in 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 Amphioxus . 157a 
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 
blood-corpuscles (. 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 
occipital condyles, and their red blood-corpuscles are not 
nucleated 158 {Mammalia). 
