558 ANURA [CH. 



three external gills on each side, but there is no trace of limbs and 

 the gill-slits are closed, each being represented by two closely 

 adherent layers of epithelium, and as the mouth does not open into 

 the alimentary canal no food is taken. Later the gill-slits become 

 open; but a flap of skin, the gill-cover, grows back from the 

 second visceral arch (the hyoid) and covers up the gill-slits and 

 the external gills. The external gills then soon disappear. The 

 two gill-covers unite with one another beneath the animal, so only 

 one little opening to the gill-chamber remains, usually on the left 

 side. The mouth has by this time opened into the alimentary 

 canal, and it is provided with two horny ridges, one above and 

 one below, besides rows of little horny prickles. The horny jaws 

 crop the water- weeds upon which the tadpole lives. 



The larva is now the well-known tadpole, with a rounded body 

 and a long flat tail, with which it swims. The limbs gradually grow, 

 but for a long time the front limbs are hidden beneath the gill- 

 cover. When they finally burst through the animal sheds its horny 

 jaws and leaves the water. For a short time the tail is retained, 

 but absorption soon removes all trace of it and the development is 

 complete. 



The Anura are divided into two main groups according to the 

 development of the tongue. In the AGLOSSA it is 

 tion aSSlfica " entirely absent and the two Eustachian tubes have a 

 common opening into the pharynx. This curious 

 group only includes two genera. In one species, Pipa americana, 

 the Surinam Toad, the eggs are emitted from the protruded oviduct 

 on to the back of the female, and here the young pass through 

 the tadpole stage enclosed in deep pockets of the moist skin. This 

 species as its popular name implies is an inhabitant of S. America. 

 In the PHANEROGLOSSA, on the other hand, the tongue is well 

 developed, being usually free behind, and in this case used to 

 flick the prey, which consists of insects, into the capacious mouth. 

 The Eustachian tubes are separate. The Phaneroglossa are divided 

 into the ARCIFERA and the FIRMISTERNIA. In the first division 

 the two epicoracoids of each side overlap (Fig. 272, B) and the two 

 halves of the pectoral girdle are slightly movable on one another ; 

 in the second they are firmly united in the middle line (Fig. 272, A). 

 The first division includes several families, but the two largest and 

 most important are those of the Toads or Bufonidae and the Tree 

 Frogs or Hylidae. 



The Toads have no teeth whatever : their wrinkled skin is beset 

 with wart-like poison glands in the upper parts, while numerous 



