178 



ZOOLOGY. 



coiled in a close spiral. The mouth is small (Fig. 225, A), 

 A with no tongue and only horny 



toothless jaws. The vertebrae of 

 the tadpole are biconcave as in 

 fishes, afterwards becoming con- 

 verted into cup-and-ball joints. 



The accompanying figures rep- 

 resent the external changes of the 

 toad from the time it is hatched 

 until the form of the adult is at- 

 tained. The tadpoles of our Amer- 

 ican toad are smaller and blacker 

 in all stages of growth than those 

 of the frog. The tadpole is at 

 first without any limbs (Fig. 226, 



Fig. 225.— Mouth and digestive A), and with two pairs of gills; 



canal of a Tadpole. A, mouth; ,-, -, • -, ■, -, , 



fe, intestine coiled on itself; c, soon the hinder pair bud out. 

 cS d /^dtoen& c 'l^d After this stage (B) is reached, 

 legs; g, rectum. ^ ie ]j 0C ]y begins to diminish in 



size. Then the fore-legs grow out (6'); and finally, as at D, 

 the tail is mostly absorbed, and at E we see the little toad 

 which hops about on the bank. 



d e 



Fiq. 226.— Different stages of the Toad. 



There are nearly 700 species of this class now living, of 

 which 101 are Xovth American. 



The Batrachians are an old-fashioned type: certain fossil, 

 extinct tailed forms were as large as whales, being over 

 thirty feet in length. 



