640 MANUAL OF ZOOLOGY. 



they are circular biconcave discs. There is only one aorta 

 the left which turns over the left bronchus, and not over the 

 right, as it does in Birds. 



5. The cavities of the thorax and abdomen are completely 

 separated from one another by a muscular partition the dia- 

 phragm or midriff. 



6. The respiratory organs are in the form of two lungs 

 placed in the thorax, but none of the bronchi end in air-recep- 

 tacles, distributed through the body, as in Birds. 



7. The embryo mammal is invariably enveloped in an am- 

 nion, and an allantois is never wanting. The allantois, how- 

 ever, either disappears at an early period of life, or it develops 

 the structure known as the "placenta." The placenta is a 

 vascular organ which serves as a means of communication 

 between the parent and the foetus, but it will be noticed more 

 particularly hereafter. 



8. In no Mammal do the visceral arches and clefts of the 

 embryo ever carry branchiae, as they do in the fishes and 

 Amphibians. 



These are the essential characters which distinguish the 

 Mammalia as a class, but it will be necessary to consider these, 

 and some other points, in a more detailed manner. 



In the first place, with regard to the osteology of the Mam- 

 mals, the following points should be noticed : 



With the exception of the Whales and Dolphins (Cetacea), 

 and the Dugongs and Manatees (Sirenia), the vertebral column 

 is divisible into the same regions as in man namely, into a 

 cervical, dorsal, lumbar, sacral, and caudal or coccygeal region 

 (see fig. 241). In the Cetacea and Sirenia the dorsal region 

 of the spine is followed by a number of vertebrae which com- 

 pose the hinder extremity of the body, but which cannot be 

 separated into lumbar, sacral, and caudal vertebrae. 



In spite of the great difference which is observable in the 

 length of the neck in different Mammals, the number of 

 vertebrae in the cervical region is extraordinarily constant, 

 being almost invariably seven, as in man. In this respect 

 there is no difference between the Whale and the Giraffe. 

 The only exceptions to this law are the Manatees (Manatus) 

 which have but six cervical vertebrae ; the three-toed Sloth 

 (Bradypus tridactylus), which is commonly regarded as posses- 

 sing nine, though competent anatomists would refer the pos- 

 terior two of these to the dorsal region ; and one of the two- 

 toed Sloths (Cholozpus Hoffmannt), which has only six cervical 

 vertebrae. 



The dorsal vertebrae are mostly thirteen in number, but they 



