GENERAL CHARACTERS OF THE VERTEBRATA. 227 



this section are included the Fishes (Class Pi.'stv:'.s),and the Frcgs, Newts, 

 and their allies (Class Amphibia). They ai'e all chai-acterised by the 

 fact that they possess gills or branchi;*, either thioughout life or 

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

 cleated red blood - corpuscles {i.e., blood-corpnscles with a centi-al 

 particle or nucleus, tig. 163, d, e), and by certain embryonic chai'ac- 

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

 they are often spoken of as the Braitclivttu Vei'tebrates. 



II. SaI'ropsida (Gr. saara, a lizard ; and opsin, appearance). — In 

 this division are tlie Birds (Class Ai:es), and the ti'ue Reptiles (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 condijle) ; 

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

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

 they possess nucleated red blood-corpuscles (tig. 1(').3, h, r), and liy 

 certain embryonic characters as well. 



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

 all the ordinary Quadrupeds ; characterised by the constant absence 

 of gills ; by the skull being jointed to the vertebral cohoun 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) ; hj having non-nucleated red 

 blood-corpuscles (fig. 163, a) : and by having special glamls — the 

 mammary glands — which secrete a special fluid — the milk — Ijy 

 which the young are nourished for a longer or shorter period after 

 birth. 



These three pirimary divisions comprise the five great classes into 

 which the Vertebnita are divided : — 



1. Pisces (Fishes). 



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



3. Reptilia (Tiue Re])tiles) 



4. Aves (Birds). 



5. Mammcdia (Quiidrupeds). 



