1 78 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



339, 342), the cranial bones are united by sutures, and 

 they are fewer than in cold-blooded vertebrates ; the 

 skull has two occipital condyles, a feature shared by 

 the amphibians ; the lower jaw consists of two pieces 

 only (often united), and articulates directly with the 

 cranium ; with four exceptions there are always seven 

 cervical vertebrae : 45 the dorsal vertebrae, and therefore 

 the ribs, vary from ten to twenty-four; the lumbar 

 vertebrae number from two to nine; the sacral from 

 three to nine, and the caudal from two to forty-six ; the 



articulating surfaces of the ver- 

 tebrae are generally flat; the 

 fore limbs are never wanting, 

 and the hind limbs only in a 

 few aquatic forms; excepting 

 the whales, each digit carries 

 a nail, claw, or hoof ; the teeth 

 (always present, save in certain 



bro-spinal nervous axis contained ] ow tribes) are USUally in tWO 



in neural tube; e, chain of sympa- ' J 



thetic ganglia; </, alimentary canal; Sets, a milk Or deClduOUS Set, 



and a permanent set, and are 



planted in sockets ; the mouth is closed by flexible 

 lips ; an external ear is rarely absent ; 46 the eyes are 

 always present though rudimentary in some burrow- 

 ing animals ; they are viviparous ; and, finally, and 

 perhaps above all, while in all other animals the 

 embryo is developed from the nourishment laid up in 

 the egg itself, in mammals it draws its support, almost 

 from the beginning, directly from the parent, and, 

 after birth, it is sustained for a time by the milk se- 

 creted by the mammary glands (Fig. 367). From 

 the first, therefore, till it can care for itself, the young 

 mammal is in vital connection with the parent. 



About twenty-one hundred species are known, inhabit- 

 ing the land, the water, and the air. 



