THE BIOLOGY OF THE FROG 



CHAPTER I 

 THE AMPHIBIA IN GENERAL AND FROGS IX PARTICULAR 



The frog is a representative of the class of vertebrate 

 animals known as the Batrachia, or Amphibia. This group 

 is intermediate in many features of structure and habit be- 

 tween the fishes and the reptiles and presents the following 

 general characters : The skin is usually smooth and moist, 

 and, in all the recent Amphibia, with the exception of a few 

 of the Apoda, entirely devoid of scales. The limbs, unlike 

 those of fishes, are typically of the five-toed or pentadactyle 

 type. The skull articulates with the first vertebra by two 

 occipital condyles and is usually composed of few bones. 

 Lungs are present except in rare instances, as in certain 

 lungless salamanders, where they have been secondarily lost. 

 The heart has three chambers, a ventricle and two auricles. 

 The aortic arches are symmetrical on the two sides. There 

 is no amnion nor allantois, although there is a homologue 

 of the latter in the urinary bladder. Most of the species 

 undergo a metamorphosis, the young living in the water and 

 breathing by means of gills. The latter disappear as the 

 lungs become functional, except in some of the lower Am- 

 phibia, in which they are retained throughout life. 



The members of this class as a rule are aquatic or semi- 

 aquatic in habit, a circumstance which suggested the name 

 of the class. Even the most terrestrial of the species gener- 



