ANIMAL KINGDOM. 121 



SUBDIVISIONS OF THE SUBKINGDOMS. 



I. Vertebrates. 



Four classes are generally recognized : — 



1. Mammals. — Species suckling their young, — a characteristic 

 -peculiar to this highest branch of the animal kingdom : all are warm- 

 blooded and air-breathing. Examples: ordinary Quadrupeds, large 

 and small, with Whales and Seals. \ 



2. Birds. — "Warm-blooded and air-breathing ; oviparous ; covered 

 with feathers, and adapted for flying. 



3. Reptiles. — Cold blooded, air-breathing ; oviparous ; skin 

 naked or covered with scales. Two divisions are here included, which 

 are often made distinct classes, — (1) Amphibians, which have gills 

 when in the young state, and lose them on becoming adults, as Frogs 

 and Salamanders ; (2) True Reptiles, which breath with lungs in both 

 the young and adult stages, as Crocodiles, Lizards, Turtles, Snakes. 



4. Fishes. — Cold-blooded; breathing by means of gills; skin 

 naked, or covered with scales. 



II. Articulates. 



The classes are three in number ; one of them — Insecteans (in- 

 cluding Insects, Spiders, and Myriapods) — aerial in resjriration ; the 

 other two, including Crustaceans and Worms, breathing by means 

 of gills, and living in water or moist earth. 



A. Respiration by lung-like cavities, or through breathing-holes 

 (spiracles) along the sides or posterior part of the body, admitting 

 air to circulate in the interior. Essentially land or aerial species. 



1. Insecteans. — (1.) Insects. — The body in three parts, — head, 

 thorax, and abdomen distinct ; only three pairs of legs. Examples : 

 the Beetle, Wasp, Fly, Butterfly. 



(2.) Spiders. — The body in two parts (in the lower division, only 

 one), the head and thorax not distinct ; four pairs of legs. Examples: 

 the Spider, Tick, Scorpion. 



(3.) Myriapods. — The body worm-like in form, the abdomen not 

 prominently distinct from the rest ; legs numerous. Example : the 

 Centipede. 



B. Respiration by means of gills, — unless the species is so minute 

 that the surface of the body is equivalent to a gill in its action. Es- 

 sentially water-species, living either in water or in moist places. 



2. Crustaceans. — The body in two parts, — the anterior called 

 the cepkalofhorax, consisting of a head and thorax, the posterior called 

 the abdomen ; locomotion by means of jointed organs. Examples : 

 the Crab, Lobster, Shrimp. 



