226 VERTEBRATE ANIMALS. 
absence of gills; by the skull being jointed to the vertebral 
column by two articulating surfaces (or condyles) ; by the fact 
that the lower jaw is composed of only two pieces, and is not 
united to the skull by means of a special bone (the guadrate 
bone) ; by having non-nucleated red blood-corpuscles (fig. 121, 
a); and by having special glands—the mammary glands— 
which secrete a special fluid—the milk—by which the young are 
nourished for a longer or shorter period after birth. 
These three primary divisions comprise the five great classes 
into which the Vertebrata are divided :— 
. Pisces (Fishes). 
. Amphibia (Frogs, Newts, &c.) 
. Leptilia (True Reptiles). 
. Aves (Birds), 
. Mammalia. 
Wm PWN 
