KINDS OF ANIMALS 485 



among the cordata, although they are not strictly vertebrate. (Ex- 

 amples. Acorn worm, lancelet, sea squirt.) The five important 

 classes of vertebrates are 



Class i Pisces (fishes). The stone hag and the lamprey are sometimes 

 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, espe- 



cially 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, and later develop lungs. Bony skeleton with two pairs of 

 appendages ; no exoskeleton. (Examples. Frog, toad, newt, sala- 

 mander, 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. Lacertilia. (Examples. Lizards, chameleons, horned toad, Gila 



monster.) 



4. Crocodilia. (Examples. Alligators, 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 casso- 

 wary, 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 



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.) 



5. Plover order. (Examples. Snipe, curlew, rail, sandpiper.) 



6. Gallinae. (Examples. Hen, turkey, guinea fowl, peacock, pheas- 



ant, partridge, ptarmigan.) 



7. Columbae. (Examples. Pigeons, doves.) 



