178 



ZOOLOGY. 



coiled in a close spiral. Tlie mouth is small (Fig. 225, J), 

 ^ 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 tlie 

 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, 

 Mouth and digestive A), and with two pairs of gills; 

 soon the hinder pair bud out. 

 After this stage {E) is reached, 

 the body begins to diminish in 



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



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



which hops about on the bank. 



Fio. 22.').- 



eanal of a Tadpole. A. mouth; 

 b, intestine coileil on itself; c, 

 liver; d. hepatic duct; e, pan- 

 creas; y, rudimentary hind 

 legs; <7, rectum. 



Fio. 326.— Different stages of the Toad. 



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

 which 101 are North American. 



The Batrachiaus are an old-fashioned type- certain fossil, 

 3xtinct tailed foi-ms were aa large as whales, being over 

 thirty feet in length. 



