656 



COMPARATIVE ANATOMY 



rudimentary limbs under the skin as has the python. The eyes are with- 

 out eye-lids. 



Class V. Aves. (Birds). 



These are closely related to the reptiles. In fact, reptiles and birds 

 are often grouped together as Sauropsida. They have a single occipital 

 condyle as do the reptiles. The heart of birds is, however, divided into 

 right and left halves. Birds are warm-blooded. There is a fusion of 

 the bones of the manus and there is the formation of a tibio-tarsus and 

 tarso-metatarsus (intratarsal joint). Feathers cover the body. 



Birds are commonly divided into Ratitae or "running birds" such as 

 the ostrich, rheas, cassowaries, etc., which lack a furcula (wish-bone) 

 and a keel (Fig. 418) to the sternum, and the Carinatae, or the "flying 

 birds." These latter have the sternum keeled and the clavicles are 

 united to form the furcula. 



There are two extinct groups which had teeth. 



Class VI. Mammalia. 



These are warm-blooded animals having a covering of hair, two 

 occipital condyles, and milk-glands in the female. 

 Mammals are divided into two sub-classes. 

 Sub-Class I. Prototheria, or egg-laying mammals. 



Order I. Monotremata, which consists of two families (Fig. 383). 



Ecliidna aciJeala (spiny ant-eater) 



Fig. 383. Monotremata. 



C. Echidna hystrix. I, lower surface of brooding female ; //, dissection showing 

 a dorsal view of the marsupium and mammary glands ; t t the two tufts of hair 

 projecting from the mammary pouches from which the secretion flows ; 6m, 

 brood-pouch or marsupium; cl, cloaca; g.m., groups of mammary glands. (A, after 

 Shipley and MacBride ; B, after Claus ; C, after Haake.) 



