CHAPTER XX 



BRANCH ARTHROPODA: CRUSTACEANS, CEN- 

 TIPEDS, INSECTS, AND SPIDERS 



I THE great branch Arthropoda includes a host of 

 "T familiar animals. It contains more species than any other 

 branch of the animal kingdom. To it belong the cray- 

 fishes, shrimps, crabs, lobsters, water-fleas, and other ani- 

 mals which compose the class Crustacea ; the centipeds 

 and thousand-legged worms which compose the class 

 Myriapoda ; the true or six-footed insects forming the class 

 Insecta, which includes nearly two-thirds of all the known 

 species of animals; and the scorpions, mites, ticks, and 

 spiders which constitute the class Arachnida. There is 

 also a fifth class in the branch Arthropoda which includes 

 a few species of animals unfamiliar to us but of great 

 interest to zoologists. 



All these varied kinds of animals have a body on the 



^ annulate or segmented type-plan, like -that shown by most 

 worms, but they differ from the worms in possessing 

 jointed appendages, used for locomotion or food taking. 

 There is typically or racially one pair of these jointed or 

 segmented appendages on each segment of the body, but 

 in all of the Arthropoda some of the segments have lost 

 their appendages. The body is covered by a firm cuticle 

 or outer body-wall called the exoskeleton. This exo- 

 skeleton serves not only to enclose and protect the soft 

 parts of the body but also for the attachment of the body 



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