462 THE CAT. [CHAP. xm. 



but incompletely divided, and the posterior nares open far forwards 

 within the mouth. There are Fallopian tubes, and in development 

 both an amnion and allantois are formed, but no placenta the 

 allantois being itself directly respiratory. There are never any 

 gills. 



Some lizards and almost all serpents are limbless, but the croco- 

 diles and alligators differ from the other reptiles in having two 

 distinct ventricles, with nares prolonged to quite the posterior end of 

 the palate. Nevertheless, even in crocodiles, there is more than one 

 aortic arch, and these arches so communicate that a mixture of 

 venous and arterial blood takes place in them. 



15. We may then say that the cat, as a MAMMAL, differs from 

 the whole class EEPTILIA in the following characters : 



(1) There is a petrous bone, and a mandible formed of two bones 



only, each directly suspended from the squamosal. 



(2) The auditory ossicles are minute, and take no part in sus- 



pending the mandible. 



(3) There are two occipital condyles. 



(4) The skin is furnished with hair. 



(5) There is a corpus callosum, and the corpora quadrigemina 



are small. 



(6) There is a cribriform plate. 



(7) All the blood passes through the lungs at each circuit. 



(8) There is no communication between the arterial and venous 



vessels outside the heart, save the capillaries. 



(9) There is but one aortic arch. 



(10) There is a complete diaphragmatic partition. 



(11) There is no intertarsal joint, the tarsus moving as one whole 



upon the tibia. 



(12) The red blood corpuscles are not nucleated, and the blood is 



warm. 



With the exception of the crocodiles and alligators, we may say 

 that the cat also differs from all reptiles : 



(13) In that it has two distinct ventricles, and 



(14) In that the posterior nares open far back within the mouth. 

 16. Turning now to the class AVES, whatever bird we may 



select as our type, will exhibit to us structural peculiarities well 

 deserving attention. The rook (Corvus) will be a good example to 

 contrast with the cat and the cat's class. Its different external 

 aspect (as compared with the cat) depends upon the following con- 

 ditions : The rook's posture, on the ground, is nearly upright, its 

 body being supported on the legs only, the pectoral limbs being 

 modified into wings. The facial part of the head is produced into a 

 long, conical, pointed beak. The region of the chest is rounded and 

 prominent. The whole frame (except the beak and legs) is clothed 

 with feathers, and it is to them that the appearance of a long tail 

 is due, for only a stumpy tail-structure is visible when they are 

 removed. The feathers are very different in different parts of the 

 body, much the longest being attached to the tail and arm, and 



