344 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



(physiologically double) or two ventricles, and the aortic 

 arches are seldom more than two. There is never a complete 

 respiratory diaphragm. The kidneys are metanephric, and 

 their ducts open into a cloaca, together with the intestine 

 and sexual ducts. The cerebral hemispheres are not united 

 by a well-marked corpus callosum, and the hypoglossal nerve 

 perforates the brain-case. The sclerotic coat of the eye is 

 supported by cartilage or bone, the cochlea is straight or 

 slightly curved, and there is a columella. The large telole- 

 cithal ova are invested in firm calcareous shells, the cleavage 

 is meroblastic, and most of the development takes place, with 

 few exceptions, outside the body of the parent. 



Class 1. Reptilia (Reptiles). 



Class 2. Aves (Columba). Warm-blooded Sauropsida 

 with fore-limbs modified into (usually) functional wings, 

 and a reduced tail-region (uropygium). There is an 

 epidermic exoskeleton, in the form of feathers. The 

 cervical vertebrae possess saddle-shaped articular surfaces, 

 the true sacral vertebrae are without expanded ribs, and 

 fuse with numerous other vertebrae to form a pseudo- 

 sacrum, and the posterior caudal vertebrae (usually) 

 unite together into a ploughshare-bone. The manus 

 never possesses more than two free carpals nor more 

 than three digits ; there is a carpo-metacarpus. The 

 ilia extend far behind and in front of the perforated 

 acetabulum, and the pubes and ischia are directed 

 backwards, the former rarely, the latter never meeting 

 in a ventral symphysis. The tarsal bones are never 

 separate, but fuse with tibia proximally and metatarsus 

 distally to form a tibio-tarsus and tarso-metatarsus ; 

 digits of pes, never more than four. The jaws are 

 covered by a horny sheath, functional teeth never being 

 present in recent forms ; a bursa Fabricii opens into 

 the cloaca. The heart is four-chambered, and the right 

 auriculo-ventricular valve muscular. There is a single 

 aortic arch, curving to the right. The lungs are immo- 

 bile, and the bronchia communicate with air sacs. The 

 right ovary and oviduct are rudimentary. The optic 

 lobes are displaced to the sides of brain. 



