THE STUDY OF AMPHIBIANS 121 



frog depends altogether on this primitive cutaneous respiration, 

 which is entirely lost in higher animals. Sometimes a newt is 

 found without lungs, apparently thriving though breathing 

 solely through the skin. It is instructive to note such cases of 

 cutaneous respiration among vertebrates, partly because this 

 method is common among invertebrates, partly because it makes 

 it clear that respiration is simply the absorption of oxygen and 

 the elimination of carbon dioxide. It matters little whether the 

 blood vessels are spread out on the walls of a lung which is open 

 to the outer world, or on the large surface of a feathery gill 

 which is washed by the water, or in the moist skin. In 

 regard to the possession of lungs, it should be noted that 

 the nostrils, through which the air enters, open into the mouth ; 

 whereas the nostrils of fishes are blind, purely olfactory sacs, of 

 no service in respiration, and without any connection with 

 the mouth (except in mud-fishes, where the conditions are 

 peculiar). 



In existing amphibians there is rarely any exoskeleton, but 

 some extinct forms had an armour of bony plates. In many 

 cases, at least, the soft skin, which is rich in glands, seems to be 

 unpalatable to various other animals, which leave amphibians 

 severely alone. 



The eggs of amphibians are small, numerous, usually pig- 

 mented, and with yolk towards one pole. They are almost always 

 laid in water, and the youthful stages are almost always 

 aquatic. 



Amphibians differ from fishes in having digits, lungs, a three- 

 chambered heart, vocal cords, a naked skin, and so on. They 

 agree with fishes in certain important respects, e.g. in having 

 gill clefts used in respiration, in having gills, in showing (in youth 

 at least) lateral sense organs like the lateral line of fishes, in 

 usually having unpaired fins (in their larval stages at least). In 

 these respects they differ from reptiles, and the usual nakedness 

 of the skin has been already referred to. Without making too 

 much of a business which is by no means so easy as it looks, it 

 may be profitable to take a trout, a newt, and a lizard, and try to 

 get the pupils to compile a statement of their resemblances and 

 differences. 



