322 lOHTHYOPSIDA. 



B. Sauropsida = Birds and Eeptiles. — There are never found 

 branchiae in either of these classes, and the embryo is ijrovided 

 ■with both an amnion and allantoic membrane. The red blood- 

 corpuscles are nucleated, and the skull always articulates with 

 the vertebral column by a single occipital condyle, whilst the 

 lower jaw has always a bone between it and the skull called the 

 quadrate lone. Diaphragm never complete. 



G. Mammalia = Mammals. — These have always two condyles 

 on the skull for articulating with the vertebral column. The 

 lower jaw, which is composed of two bones only, articulates, as 

 We saw in the skeleton of the horse, direct with the skull, 

 the quadrate bone not being present in its normal position. In 

 mammals it has become attached to the auditory apparatus. 

 Lastly, there are always developed in the female special ventral 

 glands — the mammary glands — for the nourishment of the young. 

 Diaphragm forming a complete septum. 



A. ICHTHYOPSIDA. 



I. Pisces oe Fish. 



Fish are aquatic vertebrate animals which breathe entirely by 

 d 



Pia. lOS.— The Perch (Perca JiuviatUis), aTeleostean. 



jj, Paired pectoral fins ; v, paired pelvic fins ; d and d', dorsal flns ; c, caudal fin ; 

 a, anal tin ; o, operculum ; I, lateral line. (Nicholson.) 



means of giUs (except in a few curious specially modified species, 



