GENERAL CHARACTERS OF THE VERTEBRATA. 205 



and the frogs, newts, and their allies (Class Amphibia). They 

 are all characterized by the fact that they possess gills or 

 branchiae, either throughout life or during the earlier stages 

 of their existence ; that they possess nucleated red blood- 

 corpuscles (i. e., blood-corpuscles with a central particle or 

 nucleus. Fig. 99, d, e), and by certain embryonic characters 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, appear- 

 ance). In this division are the birds (Class Aves), and the 

 true reptiles (Class Reptilia). They are characterized 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. 99, #, c\ and by 

 certain embryonic characters as well.* 



III. MAMMALIA (Lat. mamma, the breast). In this di- 

 vision are all the ordinary quadrupeds ; characterized 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. 99, 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. Fishes (Pisces). 



2. Amphibia (Frogs, Newts, etc.). 



3. Heptilia (True Reptiles). 



4. Aves (Birds). 



5. Mammalia. 



* 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. 



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