36 Vertebrata. 



the lungs, and is returned from hence into the heart. 

 It may also be noted that, during this process of 

 development in the common frog the digestive canal, 

 which in the tadpole is long and spirally coiled, 

 becomes shorter and straighter The blood of amphi- 

 bians is remarkable for the large size of the oval red 

 corpuscles which it contains, those in Proteus being 

 of an inch in diameter, those in the frog being 

 The vertebral column in the simplest of the 

 amphibians consists of rudimentary or biconcave 

 vertebrae ; in frogs (fig. 15), however, it consists of a 

 chain of a few solid disks whose surfaces fit into each 

 other by ball and socket joints. Ribs are either very 

 short or, as in frogs, absent. The skull articulates 

 to the foremost vertebra by means of two lateral arti- 

 cular surfaces which are called condyles. The skull 

 is also, as a rule, much more consolidated than the 

 skull in fishes, but resembles the latter in having, as 

 the most conspicuous bone in its base, a long ossifi- 

 cation in the membrane underlying the middle of the 

 cartilage of the base of the skull, which is known as 

 the parasphenoid bone, a bone which is rudimental or 

 absent in all higher forms. Amphibians also differ 

 from fishes in having a middle ear, closed by a tym- 

 panic membrane, and not merely the internal ear 

 cavity which constitutes the ear in fishes. Their nasal 

 cavities open posteriorly into the pharynx. They 

 have usually four limbs, which consist of parts com- 

 parable with those in higher animals, and very unlike 

 the fins in fishes. 



There are three orders of amphibians at present 

 represented by living forms on the globe. 



