ANATOMY OF PROTEUS ANGUINUS. 



such a distinct line of demarcation between these two great classes of 



Vertebrata that it would be impossible for 

 the most superficial zoologist to confound 

 one with the other, or to be for a single 

 moment at a loss in attempting to assign to 

 any creature belonging to either of these 

 divisions of the animal world its proper posi- 

 tion; indeed, to mistake an air-breathing 

 reptile for a fish properly so called, would 

 appear to be an error which the most igno- 

 rant naturalist could hardly be in danger of 

 committing. 



(1941.) We have, however, again and 

 again had opportunities of observing how 

 nearly animals of neighbouring classes ap- 

 proximate each other, not only in their out- 

 ward form, but in their anatomical con- 

 struction, and in considering this portion 

 of our subject we shall have another most 

 striking illustration of this great law in 

 zoology. 



(1942.) The perfect and typical Reptile, 

 as the Lizard, the Tortoise, and the Ser- 

 pent, breathes air, and air only, and is there- 

 fore only provided with lungs adapted to 

 this kind of respiration ; but the Perenni- 

 branchiate Amphibia, possessing both lungs 

 and gills, participate to a greater or less de- 

 gree in the characters of Fishes ; so that in 

 some, as, for example, in the Lepidosiren 

 (fig. 328), so near is the approximation, 

 that it becomes almost impracticable for the 

 most accomplished anatomist precisely to 

 determine whether the animal ought rather 

 to be called a reptile or a fish ; and lastly, 

 in the Batrachian Amphibia, as we have al- 

 ready seen, we have the same animal gra- 

 dually changed from a fish into a complete 

 and perfect reptile. 



(1943.) In considering the apparatus pro- 

 vided for circulation and respiration in the 

 animals comprised in the class before us, we 

 shall therefore first describe the organiza- 

 tion of these viscera in Reptiles furnished 

 with lungs only ; secondly, of those having 



Fig. 340. 



Anatomy of Proteus anguin us. 



