158 AMPHIBIA 



the paired limbs of fishes arejins, fan-like structures of which the 

 skeleton consists of rays radiating from the girdles to which they 

 are attached, while those of Amphibia, like those of all terrestrial 

 vertebrates, are legs, jointed cylindrical structures resembling 

 fins only in their terminal parts, the feet, which are supported 

 by five rays termed the digits. The terrestrial limbs have 

 been described as tetrapodous and pentadactyle, but neither of 

 these epithets is particularly appropriate ; tetrapodous merely 

 means four-footed and the number of limbs is the same in 

 fishes, while pentadactyle, although it expresses a general 

 characteristic of the terrestrial limb, refers only to the foot or 

 terminal portion which is most similar to the fin of a fish. 

 More appropriate terms to express the conditions to which the 

 two types of limb are adapted, and thus indirectly their structure, 

 would be Jiydrobatous or going in water, and geobatous or 

 moving on land. The fin of the fish has also been termed the 

 ichthyopterygium, and the leg of terrestrial vertebrates the 

 cheiropterygium, terms which mean fish-limb and hand-limb, 

 but the latter term is obviously inaccurate as the word hand 

 does not suggest the leg of an animal. The important fact is 

 that there is an essential difference in structure between the 

 limbs of fishes and those of terrestrial vertebrates and this dif- 

 ference is as well indicated by the English words fin and leg as 

 by any others. Many Amphibia during part or all of their 

 lives possess median unpaired fins, but these are never, like 

 those of fishes, supported by skeletal fin-rays. 



These differences distinguish the Amphibia from fishes but 

 not from other classes of terrestrial Vertebrates. From these 

 they are completely separated by the absence of the amnion in 

 the embryo and the absence of the allantois — an outgrowth of the 

 hind-gut of the embryo which absorbs oxygen, and thus serves 

 instead of a lung during embryonic development. The allantois 

 cannot be said to be entirely wanting, for the urinary bladder 

 of the Amphibia seems to be its representative ; but this 

 bladder has nothing to do with the respiration of the embryo. 

 These differences however are of little use in distinguishing 

 adult specimens. All existing Amphibia differ from reptiles in 

 the absence of horny epidermic scales or scutes, and in the 

 presence of glands which render the skin soft and moist ; in 

 the order Apoda or limbless Amphibia dermal scales are 



