THE STUDY OF AMPHIBIANS 133 



cleft, sometimes called the " complementary spiracle." The 

 mouth is somewhat lamprey-like, with horny " teeth " on the 

 circular lip, and the jaws still bear the horny beaks somewhat 

 like those of cuttle-fishes. Inside the mouth there are many 

 long papillae, possibly " taste organs." The gill-arches still bear 

 delicate branching filaments to the outer convex side, and show 

 a membranous filtering sieve on the inner concave side, but the 

 gill-system is in process of disappearing, for the lungs have now 

 developed. In an intricate way the circulation changes from 

 being very like that of fish towards the adult amphibian arrange- 

 ments. The tadpoles come often to the surface and take gulps 

 of air ; they are learning to use their lungs. If they are pre- 

 vented by a screen of gauze from coming to the surface they 

 may become large tadpoles, and may be kept in this state for 

 over a year. In the north, when the summer is very cold, 

 the winter may come before the tadpoles have changed into small 

 frogs. 



The Metamorphosis. Some three months or so after hatching 

 the tadpoles change into little frogs, and this metamorphosis 

 involves much reconstruc- 

 tion. The tail degenerates 

 and is absorbed, a process 

 analogous to inflammation 

 taking place. The wander- 

 ing amoeboid Cells Or phagO- FlG< 4 6 - A tadpole undergoing metamorphosis 

 f ., , , more than two months after hatching. The 



cytes Of the body carry ua is beginning to shorten, the gills ari being 

 the debris Of the tail tO be absorbed and the gill-clefts closed up, the horny 



utilised in construction in J aws are lost > the e y es become more prominent, 



. i -i i A , and so on. 



the body. A popular im- 

 pression that tadpoles shed their tails is still widespread, but 

 entirely erroneous. Loose tails sometimes found lying on the 

 mud of the pool where tadpoles abound have been cut off 

 by the powerful jaws of the larvae of the large water-beetle 

 Dytiscus marginalis. But the changes are equally marked 

 at the anterior end of the body. The horny armature of the 

 jaws and lips is shed ; the lips are absorbed and the mouth 

 becomes wide ; the eyes become at last free and get movable 

 lids ; the tongue becomes muscular enough to move ; the in- 



