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 

 r . 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. Columbas. (Examples. Pigeons, doves.) 



