GENERAL CHARACTERS OF THE VEETEBRATA. 227 



this section are included the Fishes (Class Ptgce^Xand the Frogs, Newts, 

 and their allies (Class Ampkihia). They are all characterised by the 

 fact that they possess gills or branchiae, either throughout life or 

 during the earlier stages of their existence ; that they possess nu- 

 cleated red blood - corpuscles {i-c, blood-corpuscles with a central 

 particle or nucleus, fig. 163, d, e), and by certain embryonic charac- 

 ters as well. From the temporary or permanent possession of gills 

 they are often spoken of as the Branchiate Vertebrates. 



II. Sauropsida (Gr. saura, a lizard ; and opsis, appearance). — In 

 this division are the Birds (Class Aves), and the true Eeptiles (Class 

 Reptilia). They are characterised by the fact, that at no time of 

 their life are they ever provided with gills ; that the skull is jointed 

 to the vertebral column by a single articulating surface (or condyle) ; 

 that the lower jaw is composed of several pieces, and is united to the 

 skull by means of a special bone (called the os quadratum) ; that 

 they possess nucleated red blood-corpuscles (fig. 163, 5, c), and by 

 certain embryonic characters as well. 



III. Mammalia (Lat. mamma, the breast). — In this division are 

 all the ordinary Quadrupeds ; characterised by the constant 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 quadrate bone) ; by having non-nucleated red 

 blood-corpuscles (fig. 163, 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 : — 



1. Pisces (Fishes). 



2. Amphibia (Frogs, Newts, &c.) 



3. Reptilia (True Reptiles) 



4. Aves (Birds). 



5. Mammalia (Quadrupeds). 



