CLASSIFICATION OF AMPHIBIA. 605 
of that transition from aquatic to aerial respiration which 
must have marked one of the most momentous epochs in 
the evolution of Vertebrates. 
Fic. 327.—Life history of a frog.—After Brehm. 
1-3, Developing ova; 4, newly hatched forms hanging to water- 
weeds; 5, 6, stages with external gills; 7-10, tadpoles during 
emergence of limbs ; 11, tadpoles with both pairs of limbs appa- 
rent ; 12, metamorphosis to frog. 
CLASSIFICATION OF AMPHIBIA 
Order ANURA or ECAUDATA 
The adults have no tail or external gills or open gill-clefts, There 
are always four limbs. z 
Sub-order Phaneroglossa.—Tongue present; the Eustachian tubes 
open separately into the pharynx. : 
Series A. Arcifera (see p. 584), ¢.g. the toothless toads (Bz/fo) ; 
the tree-frogs (Hy/a), with adhésive glandular discs 
on the ends of the digits; the obstetric frog (Alyzes) ; 
Bombinator, Pelobates, and others. 
Series B, Firmisternia (see -p. 584), the frogs proper (Ranidz), 
eg. the gtass-frog (R. temporaria), the edible 
frog (R. esculenta), the N. American bull-fiog 
(R. catesbiana), sometimes 8 in. in length, and 
with a sonorous croak. 
