6 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



A higher stage is represented by the lancelet {Amphioxus), 

 sole type of the class Cephalochordata. This animal is 

 like a far-oflf prophecy of a iish, but is perhaps in some 

 respects degenerate. 



Approaching Fishes, but still far from them, are the Round- 

 mouths or Marsipobranchs, Cyclostomata, e.g., the hag 

 (Myxine) and the lamprey (Petromyzon). They have no 

 limbs, nor . scales, nor true jaws, and their respiratory 

 arrangements are peculiar. 



At last we reach the Fishes (Pisces), finned, scaly, and 

 jawed, including four great sets : — 



1. Elasmobranchii. Gristly fishes, e.g.. Sharks and skates. 



2. Ganoidei. Heavily armoured fishes, e.g.. Sturgeon and bony- 



pike. 



3. Teleostei. Modern bony fishes, e.g. , Cod and salmon. 



4. Dipnoi. Double-breathingMud-fisheSjinwhichtheswim-bladder 



acts as a lung, and aids the gill. Ceratodus, Protopterus. 



The double-breathing mud-fishes approach in some degree 

 towards the next class — the Amphibia, including, besides the 

 extinct Labyrinthodonts, the following orders : — 



1. Gymnophiona, earthworm-like subterranean amphibians, e.g., 



Ccecilia. 



2. Urodela, tailed animals, like the newts and salamanders. 



3. Anura, tail-less forms, frogs and toads. 



The Amphibians are soft-skinned, and have fingered 

 limbs. In their youth they breathe by gills, and they may 

 retain these throughout life, but whether they keep their 

 gills or lose them, they always acquire true lungs. There are 

 not many thoroughgoing distinctions between Amphibians 

 and Fishes, and the two classes are ranked together as 

 Ichthyopsida. 



What the "worms" are among Invertebrates, the 

 Reptilia are among Vertebrates, an assemblage of classes. 

 Unfortunately, however, most of these are extinct. In fact 

 only five classes are now represented : — 



1. Chelonia. Tortoises and turtles. 



2. Proterosauria. Sphenodon or Hatteria, a remarkable New- 



Zealand "lizard." 



3. Lacertilia. Lizards. 



4. Ophidia. -Snakes. 



5. Crocodilia. Crocodiles and Alligators. 



