ANIMAL KINGDOM. 121 



SUBDIVISIONS OF THE SUBKINGDOMS. 



I. Vertebrates. 



The five classes are : — 



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

 peculiar to this highest branch of the animal kingdom ; breathing by 

 lungs ; having a heart of four cavities : as ordinary Quadrupeds, with 

 Whales and Seals. 



2. Birds. — Oviparous; breathing by lungs ; heart of four cavities ; 

 covered with feathers, and adapted for flying. 



3. Reptiles. — Oviparous ; breathing by lungs ; a heart of three or 

 four cavities ; naked or covered with scales : as Crocodiles, Lizards, 

 Turtles, Snakes. 



4. Amphibians. — Oviparous ; breathing when young by gills, and 

 afterward by lungs alone ; a heart of three cavities ; naked or covered 

 by scales : as Frogs and Salamanders. 



5. Fishes. — Usually oviparous ; heart usually of two cavities ; 

 breathing by gills ; naked, or covered with scales. 



II. Articulates. 



The Articulates include (1) the Arthropods (so named from <*q6qov 

 joint, and novg foot), or those having jointed legs ; and (2) Worms 

 (or Annelids), those without them. 



I. Arthropods. The Arthropods are divided into two groups 

 based on adaptation to life in the air or in the water ; and, further, 

 into classes according to structure. 



A. The Terrestrial, which have lung-like cavities, or breathing- 

 holes (spiracles), and a system inside for air-circulation. In this 

 division there are — 



1. Insects. — Having the body in three parts, that is, a distinct 

 head, thorax, and abdomen ; and only three pairs of legs : as, Beetles, 

 Flies, Butterflies. 



2. Spiders. — Having the body in two parts, or, in the lowest, only 

 one, the head and thorax not separate segments. 



3. Myriapods. — Having a worm-like form, and numerous pairs 

 of legs. 



B. The Aquatic, which have gills for aquatic respiration, or 

 else some other part of the body serving this purpose. Here 

 belong — 



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

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

 abdomen : as, Crabs, Lobsters, Shrimps. 



