AMPHIBIA CHAP. 



So far as numbers of living species are concerned, the 

 Amphibia are the least numerous of the Vertebra ta. There are 

 about 40 limbless, burrowing APODA ; 100 UKODELA or tailed 

 two- or four-footed newts, and about 900 ANURA, or tailless, 

 four-footed frogs and toads ; in all some 1000 different species. 

 Few, indeed, in comparison with the 2700 Mammals, 3500 

 Eeptiles, nearly 8000 Fishes, and almost 10,000 Birds. But 

 we shall see that the Amphibia have not only " had their day," 

 having flourished in bygone ages when they divided the world, so 

 far as Vertebrata were concerned, between themselves and the 

 Fishes, but that they never attained a dominant position. Inter- 

 mediate between the aquatic Fishes and the gradually rising terres- 

 trial Eeptiles they had to fight, so to speak, with a double front 

 during the struggle of evolution, until by now most of them have 

 become extinct. The rest persist literally in nooks and corners 

 of the teeming world, and only the Frogs and Toads, the more 

 recent branch of the Amphibian tree, have spread over the whole 

 globe, exhibiting almost endless variations of the same narrow, 

 much specialised plan. The greatest charm of the Anura lies in 

 their marvellous adaptation to prevailing circumstances ; and 

 the nursing habits of some kinds read almost like fairy-tales. 



Characters of the Amphibia. 1 



1. The vertebrae are (a) acentrous, (6) pseudocentrous, or (c) notocentrous. 



2. The skull articulates with the atlas by two condyles which are formed 



by the lateral occipitals. For exceptions see p. 78. 



3. There is an auditory columellar apparatus, fitting into the fenestra ovalis. 



4. The limbs are of the tetrapodous, pentadactyle type. 



5. The red blood -corpuscles are nucleated, biconvex, and oval. 



6. The heart is (a) divided into two atria and one ventricle, and (6) it has 



a conus provided with valves. 



7. The aortic arches are strictly symmetrical. 



8. Gills are present at least during some early stages of development. 



9. The kidneys are provided with persistent nephrostomes. 



10. Lateral sense-organs are present at least during the larval stage. 



11. The vagus is the last cranial nerve. 



12. The median fins, where present, are not supported by spinal skeletal rays. 



13. Sternal ribs and a costal or true sternum are absent. 



14. There is no paired or unpaired medio- ventral, copulatory apparatus. 



15. Development takes place without amnion and allantois. 



None of these characters is absolutely diagnostic, except 1 (c), and this 

 applies only to the Anura and most of the Stegocephali. 



1 References to explanations of the terms used below will bo found in the index. 



