112 NATURE STUDIES. 



PART II. FROGS AND TADPOLES. 



HAVING in my previous paper tried to show that the 

 mud-fishes were veritable links between the fish-class 

 and the frog-class, we may now turn to the history of 

 the latter group itself, by way of showing how, within 

 its own limits, gaps and gulfs have been bridged in 

 nature's own way. The history of a frog is in itself 

 an interesting study. It begins life as a tadpole, and 

 lives, as most readers know, a perfectly fish-like 

 existence. It is fish-like in form ; its heart is two- 

 chambered, and thus resembles that of the fish, and 

 it breathes at first by outside gills. By-and-bye a 

 broad fold grows over the gills, and ultimately covers 

 them ; whilst internal gills grow from the gill-arches. 

 Meanwhile, the tadpole has been cropping the water- 

 weeds by means of the horny jaws with which it is 

 provided, and has been digesting its food within the 

 long and spiral intestines which is the right and heri- 

 tage of the vegetable feeder. Soon, however, the 

 hind legs, which in the frogs and toads are the first 

 to appear, are developed ; and these are in turn suc- 

 ceeded by the front limbs. Lungs begin also to 

 grow, as all lungs do, namely, in the form of two sacs 

 or bags from the hinder or lower wall of the gullet. 

 At this stage, the likeness of the frog to the fish has 

 disappeared, and it closely resembles one of the 

 common tailed "efts" or newts, which are familiar 

 denizens of our ponds and pools. If it had retained 



