GENERAL CHARACTERS OF THE VERTEBRATA. 225 
are variously divided into great primary sections by different 
writers, and all of these divisions have more or less merit. 
Here, however, the classification proposed by Professor Huxley 
will be followed, and it is not necessary to enter into any consi- 
deration of the others. It has also been thought advisable to 
give in this place a brief account of the leading characters which 
separate these divisions from one another, though it is not to be 
expected that the learner will be able to appreciate the full 
value of these characters till he has completed his study of the 
Vertebrata as a whole. 
The Vertebrata are divided by Professor Huxley into the fol- 
lowing great divisions :— 
I. ICHTHYOPSIDA (Gr. zchthus, a fish; and ofsis, appearance). 
—In this section are included the fishes (Class Pisces), and the 
frogs, newts, and their allies (Class Amphibia). They are all 
characterised by the fact that they possess gills or branchie, 
either throughout life or during the earlier stages of their exist- 
ence; that they possess nucleated red blood-corpuscles (2.2., 
blood-corpuscles with a central particle or zucleus, fig. 121 sy CN, 
and by certain embryonic characters as well. From the tem- 
porary or permanent possession of gills, they are often spoken 
of as the Branchiate Vertebrates. 
II. SAUROPSIDA (Gr. saura, a lizard ; and ofsis, appearance). 
—In this division are the birds (Class Aves), and the true rep- 
tiles (Class Repti/ia). 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 articulat- 
ing 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 guadratum; that they possess nucleated 
red blood-corpuscles (fig. 121, 4, 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 
Recent»researches have led to the belief that the appearance of nuclei in the red 
blood-corpuscles of the Oviparous Vertebrates is due to changes taking place after 
death, and that these structures are not present during life, 
P 
