THE SORTING OF PLANTS AND ANIMALS 87 



included among the cordata, although they are not strictly verte- 

 brate. {Examples. Acorn worm, lancelet, sea squirt.) The five 

 important classes of vertebrates are as follows : 

 Class i — Pisces (fishes). The stone hag and the lamprey are some- 

 times called fishes, though they are distinct in having a 

 round mouth (no jaws) and no fins or scales. They never 

 develop bones, the skeleton remaining cartilaginous. There 

 are four orders of true fishes : 



1. Cartilaginous fishes. Gill slits not covered; ''skin teeth." {Ex- 



amples. Skates, torpedoes, sharks.) 



2. Armored fishes {Ganoidei). Large, bony scales in the skin, es- 



pecially about the head. In former times this order was 

 very numerous. {Examples. Sturgeon and gar pike.) 



3. Bony fishes {Teleostei). {Examples. Salmon, herring, perch, 



cod, flounder, etc.) 



4. Mud fishes {Dipnoi). Fishes with lunglike structures. Only 



three living representatives, all in the southern hemisphere. 



Class 2 — Batrachians (amphibia). Breathe by means of gills in 

 early stages, famihar to us as tadpoles, and later develop 

 lungs. Bony skeleton with two pairs of appendages ; no 

 exoskeleton. {Examples. Frog, toad, newt, salamander, 

 mud puppy, hellbender.) 



Class 3 — Reptilia. Wholly air-breathers; plates or scales in the 

 skin. Four orders are usually recognized : 



1. Chelonia. {Examples. Turtles and tortoises.) 



2. Serpents. {Examples. Snakes, adders, cobras.) 



3. Lacertiha. {Examples. Lizards, chameleons, horned toad, Gila 



monster.) 



4. Crocodilia. {Examples. AlHgators, crocodiles.) 



Class 4 — Aves (birds). Warm-blooded; exoskeleton of feathers; 

 front limbs wings ; tendency for the bones to fuse ; air 

 spaces in bones ; no diaphragm ; eggs with limy shells. 

 Living species of birds may be divided conveniently into 

 the running birds (ostriches, the cassowary, and the emu) 

 and the flying birds. The latter include two groups of 

 orders — the water birds and the land birds. Some of 

 the important orders are as follows : 



1. Anseres. {Examples. Swans, ducks, geese.) 



2. Longipennes. (Examples. Gulls, petrels, terns.) 



3. Pygopodes. {Examples. Loons, grebes, auks.) 



4. Heron order. {Examples. Storks, ibis, bittern.) 



