INTRODUCTION 3 



but the turtles and alligators spend much of their existence in the 

 water; the lizards are in many cases arboreal; and the snakes 

 live in almost every conceivable environment. They -are all 

 called cold-blooded vertebrates because their body temperature 

 varies with that of the surrounding medium and may drop to 

 the freezing point. They possess lungs, and in most cases are 

 covered with an armor of scales of bony plates. 



The most familiar AMPHIBIA (Chap. XVIII) are the frogs, toads, 

 and salamanders. They pass the first part of their lives in the 

 water, at which time they breathe by means of gills; but later 

 they become air-breathers, and many of them leave the water 

 and live on land. In form certain AMPHIBIA resemble reptiles, 

 but they usually do not possess scales and are anatomically quite 

 different. They are cold-blooded. 



The common fishes are members of the group PISCES (Chap. 

 XVII). They are cold-blooded animals, usually covered with 

 scales, and spend their entire existence in the water. They pos- 

 sess gills for breathing, and swim about by means of fins. Some 

 of them, like the sea-horse (Fig. 398), are so modified as to be 

 hardly recognizable as fish; others, called lung- fishes, are able 

 to breathe out of water. 



Belonging to the vertebrate series, but lower in the scale of 

 life than the common fishes, are two groups of fishlike animals 

 that are comparatively little known. These are the ELASMO- 

 BRANCHII, or sharks and rays (Chap. XVI), and the CYCLOSTO- 

 MATA, or lamprey-eels and hagfishes (Chap. XV; Fig. 352). 



(2) The Arthropoda. The crayfishes, centipedes, insects, and 

 spiders are among the commonest ARTHROPODA (Chap. XIII). 

 All of these animals have jointed appendages, and their bodies 

 are divided into a number of segments which are arranged in a 

 single row and are modified for various purposes. An outer 

 covering of a yellowish substance called chitin gives firmness to 

 the body and also serves as a protection from mechanical injury. 



The ARACHNIDA are the spiders, scorpions, mites, ticks, etc. 

 They may usually be distinguished from other ARTHROPODA, by 



