GENERAL CHARACTERS 167 



posterior part of the palate, the tympanic cavities are wide but 

 the tympanic membrane is not distinct from the rest of the 

 skin. The second, third and fourth vertebrae carry long ribs. 

 The two halves of the pectoral girdle meet and partly unite in 

 the middle line. The Aglossa have been regarded as very 

 primitive Anura but according to Gadow they have few primi- 

 tive characters, such as the ribs and the presence in the tadpoles 

 of paired opercular apertures ; most of the characters are 

 specialisations in adaptation to an aquatic mode of life, the 

 original evolution of the Anura having been due to adaptation 

 to terrestrial life. Xenopus or Dactylethra is a genus with 

 several species confined to Africa. It is distinguished by a 

 small tentacle below the eye, a row of apertures of cutaneous 

 tubes along each side of the dorsal surface, tubes probably 

 similar to the dermal sensory tubes of tadpoles and fishes, and 

 claws on the first three toes of the hind-foot, all the toes of this 

 foot being fully webbed. The clawed toad, Xenopus Iczvis, is 

 entirely aquatic in its habits. (Plate XV., A, B.) It lives well in 

 captivity and the tadpoles have been hatched in England. The 

 tentacles begin to sprout out on the sixth day after hatching and 

 grow to a considerable length becoming much reduced during the 

 metamorphosis. It is possible that these tentacles are homologous 

 with, or answer to, organs called balancers which are developed 

 above the angle of the jaw in the larvae of Triton, Amblystoma, 

 and other Urodela, and with the tentacular organs of the Apoda. 

 The only species of the Aglossa in South America is the cele- 

 brated Surinam Toad, Pipa americana. In this creature the 

 head is depressed and triangular, the eyes very small, cutaneous 

 flaps are situated on the upper lip in front of the eye and at 

 the angles of the mouth ; the skin is covered with small pointed 

 tubercles, the toes of the fore-foot are slender and free, terminat- 

 ing in star-shaped tips, those of the hind-foot are fully webbed. 

 The eggs are embedded in the skin of the back of the female, 

 and the young pass through the whole of their metamorphosis 

 in this position. 



III. APODA, OR LIMBLESS AMPHIBIA 



This order includes a small but remarkable group of limb- 

 less, elongated Amphibia known as Caecilians, or Gymno- 

 phiona, which live and burrow in the soil like earthworms, 



