4 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



Thus Professor Huxley grouped Birds and Reptiles together 

 as Sauropsida; Amphibians and Fishes together as Ich- 

 thyopsida — for reasons which shall be afterwards stated. 

 Amphibians mark the transition from aquatic life, habitual 

 among Fishes, to terrestrial life, habitual among Reptiles, for 

 while almost all Amphibians have gills — in their youth at 

 least — all the adults have lungs, and some retain the gills as 

 well. In having limbs which are fingered and toed, and 

 thus very different from fins, they resemble Reptiles. But 

 the two fcetal membranes characteristic of the embryonic life 

 of higher Vertebrates are not present in Amphibian embryos, 

 and the general absence of an exoskeleton in modern forms 

 is noteworthy. 



Fishes. 



The members of this class are as markedly adapted to 

 life in the water as birds to life in the air. The tail usually 

 forms the locomotor organ, and the limbs are fins. There 

 are also unpaired median fins supported by fin rays. All 

 have permanent gills borne by bony or gristly arches. 

 There is an exoskeleton of scales, and the skin also bears 

 numerous glandular cells and sensory structures. 



In many ways Fishes are allied to Amphibians, especially 

 if we include among Fishes three peculiar forms, known as 

 Dipnoi, which show hints of a three chambered heart, and 

 have a lung as well as gills. Other Fishes have a two 

 chambered heart, containing only impure blood, which is 

 driven to the gills, whence, purified, it passes directly to the 

 body. 



Apart from the divergent Dipnoi, there are three orders 

 of Fishes : — the cartilaginous Elasmobranchs, such as shark 

 and skate ; the Ganoids, such as sturgeon and bony pike ; 

 and the Teleosteans or bony fishes, such as cod, herring, 

 salmon, eel, and sole. 



Priniitive (?) J'erfebrates. 



Under this title we include (i) the class of Round- 

 mouths or Cyclostomata ; (2) the class of which the lancelet 

 or Amphioxus is the only adequately known type ; (3) the 

 class of Tunicates, some of which are called sea-squirts • 

 and (4), with much hesitation, several strange forms, 



