204 Elementary Zoology. 



are covered by an operculum. A swim-bladder is always 

 present, and is unpaired, except in Polypterus, where it is even 

 lung-like in texture, as is the unpaired swim-bladder of Lepi- 

 dosteus and Amia (two American Ganoids). 



4. Dipnoi. — The Dipnoi, or lung-fishes, form a limited 

 group (at the present day) with but three species, the 

 •Ceratodus of Queensland, the Protopterus of Africa, and 

 Lepidosiren of America. They are more akin to Amphibia 

 than are any other fishes. The body is scaly, and the skeleton 

 is largely cartilaginous. The viscera are somewhat shark-like. 

 The intestine has a spiral valve, and there is a cloaca. The 

 swim-bladder is single in Ceratodus, double in the others ; it 

 functions as a lung, and the gills are somewhat reduced. They 

 have the posterior nares (as in jto fish, but in Amphibians and 

 all higher Vertebrates) ; the heart is three-chambered. 



Sub-class 3. Amphibia. — By some the Amphibia are 

 united with the fishes to form a group, Ichthyopsida. The 

 only positive differences that distinguish them have been 

 already mentioned. They contrast with the higher Verte- 

 brate by the fact that the larva always has gills,' which may 

 be persistent throughout life, but in that case in conjunction 

 with lungs. The skull is more or less ossified with two 

 occipital condyles for articulation with the vertebral column. 

 There is no spiral valve, but the intestine opens into a cloaca ; 

 the heart is three-chambered. As in Pisces, there are ten pairs 

 of cranial nerves. The Amphibia are divisible into (i) the 

 extinct Stegocephali, including the Labyrinthodonts, which 

 were tailed, and had an extensive dermal armature of plates ; 

 (2) the Caecilia, snake-like Amphibians, of underground habit, 

 with scales ; (3) Urodela, tailed Amphibians, with usually per- 

 sisting gills, e.g. newt, salamander, axolotl ; (4) Anura, tailless 

 Amphibians, without gills, partly terrestrial in habit— the frogs 

 and toads. 



Sub-class 4. Reptilia. — The Reptiles agree with the 

 Amphibia and with all the higher Vertebrates in possessing 

 the cheiropterygium, which, however, has partly reverted to 



' With a few exceptions. Thus the tree-frogs leave the egg complete 

 frogs. 



