542 UEODELA [CH. 



the closing of the gill-slits and the shrivelling of the external gills 

 the adult state is attained. 



It has been recently pointed out that the development of the 

 Newt's fore-limb may afford a hint as to how the pentadactyle limb 

 was evolved from the fin of a fish. Prof. Broom has made the 

 plausible suggestion that this took place by the gradual freeing of 

 the anterior rays from the fin-membrane so that the limb was used 

 for a time both to walk on and swim with, exactly as occurs in the 

 pelvic fin of the modern gurnard (Trigla) where the anterior rays 

 are freed from the membrane and are used by the fish to walk on 

 the bottom. This freeing of the rays must have been a progressive 

 process the fin-membrane shrinking as it was less and less used. 

 The fingers of the Newt's hand appear one after the other and the 

 oldest known foot-print of a land animal which is found in the 

 Devonian rocks shows traces of only two well-developed toes. 



The Urodela have for a long time been divided into two main 

 groups, according to the presence during adult life of gill-slits 

 and gills. Huxley thus divided them into ICHTHYOIDEA arid SALA- 

 MANDROIDEA, and we shall adopt this mode of classi- 

 fying Urodela although we fully admit that the group 

 Ichthyoidea does not represent a single stem but probably several 

 stems in the same condition. It is advisable to base classification 

 on blood relationships when this is clear and undoubted, and this is 

 possible in the case of the larger groups. But to follow out blood- 

 felationship in minute detail is a very speculative matter in which 

 the opinions of zoologists differ sharply, and if classification is to be 

 continually altered in relation to such speculations no finality will 

 b.e obtained and great inconvenience will result. Huxley's Ichthy- 

 oidea are those which retain throughout life gill-slits or external 

 gills or both. Invariably the limbs are reduced in size, the animals 

 rarely if ever leaving the water. In one case the hind-limbs have 

 totally disappeared. 



North America is the great head-quarters of the Ichthyoidea. 

 tylenopoma (Cryptobranchus) retains one gill-slit throughout life. 

 This animal attains a length of 18 inches. It is fairly common on the 

 Mississippi and its tributaries. An allied species found in Japan, 

 and attaining a length of two feet, is the largest living Amphibian. 



Amphiuma is a snake-like animal about 18 inches long, with 

 one gill-slit. It is found in the same region as Menopoma. The 

 limbs are exceedingly rudimentary, each having only two toes. 

 . Necturus, the _ Mud-puppy, has small but well-developed limbs. 



