152 ANIMAL KINGDOM. 



of a series of vertebrae, and is hence composite in the skeleton 

 but essentially simple in the nervous system. 



V. Protozoans. — Besides the species above included, there are 

 others, of extreme simplicity of structure, which are sometimes re- 

 ferred to the Eadiates and sometimes to a separate group, called 

 Protozoans (from rtpato^, first, and £wov, animal). They embrace some 

 of the microscopic organisms or animalcules, and also the Sponges. 

 They have, in general, no proper mouth or stomach. 



Eadiates, Mollusks, Articulates, and Protozoans are Invertebrates. 



Subdivisions or the Sub-Kingdoms. 



[Details relating to the higher groups are given as follow : Fishes, 

 p. 277 i Reptiles, p. 343 ; Mammals, p. 421.] 



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, as the Crocodile, Lizard, Frog. 



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 respiration ; the 

 other two including Crustaceans and Worms, which breathe by means 

 of gills, and live in water or moist earth. 



A. Respiration 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, — a head, tho- 

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

 Beetle, Wasp, Fly, Butterfly. 



(2). Spiders. — The body in two parts, the head and thorax not dis- 

 tinct; four pairs of legs. Examples: the Spider, Tick, Scorpion. 



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

 nently distinct from the rest ; legs numerous. Examples : the Cen- 

 tipede. 



