34O ZOOLOGY 



tween the auricles in man is not completed until a late 

 stage of development, and sometimes the opening, 

 called the foramen ovate, does not close at all. In such 

 cases part of the venous blood passes to the left side of 

 the heart without going through the lungs, and con- 

 sequently the blood fails to receive enough oxygen and 

 the complexion is bluish. Fortunately such failures to 

 complete development are very rare. 



All warm-blooded animals have two auricles and two 

 ventricles. The division of the auricles begins earlier, 

 in the amphibians ; while the crocodiles, among reptiles, 

 have two ventricles. 



stages of 10. The principal stages in the evolution of the verte- 



vertebrate b ra tes may be summed up as follows : 



evolution J 



a. Development of brain and cartilaginous skull, 

 with paired eyes. (Cyclostomes.) 



b. Development of cartilaginous skeleton, with well- 

 formed vertebrae, pectoral and pelvic arches, and paired 

 fins ; also paired nostrils. (Elasmobranchs, sharks and 

 rays.) 



c. Development of bony skeleton and scales, also air 

 bladder. (Bony fishes.) 



d. Development of limbs for terrestrial locomotion, 

 with five (or fewer) digits ; development of lungs in 

 adult stage, for breathing air. (Amphibians.) 



e. Development of eggs with hard shells, which could 

 be laid on land ; elimination of early aquatic stages. 

 (Reptiles.) 



/. Warm blood, developed independently in birds and 

 mammals. This necessitated a body covering of hair or 

 feathers, or (as in the porpoise) a thick layer of fat be- 

 neath the skin. There was also developed a heat-regu- 

 lating mechanism, involving the blood system and sweat 

 glands with suitable nerve control. 



