CHAPTER XIV 

 THE ARTHROPODS 



309. Animals with Jointed Legs. This is the meaning 

 of the name Arthropoda as applied to the group of animals 

 including such forms as crayfishes, crabs, spiders, centipedes, 

 and insects. More than half of the existing species of 

 animals are arthropods, and of insects alone there are more 

 species than in all the other groups of animals taken together. 



There are four different types of common arthropods in- 

 cluded in the examples named above, and each of these types 

 represents a group known as a class. The important classes 

 are Crustacea (e.g., water-fleas, crayfishes, lobsters, crabs, 

 sow-bugs) ; Arachnida (spiders and scorpions) ; Myriopoda, 

 (thousand-legs and centipedes) ; and Insecta (beetles, 

 grasshoppers, butterflies, flies, cockroaches, etc.). We shall 

 examine examples of each of these types of arthropod animals. 



CRUSTACEANS 



310. Structure of a Crayfish, or Lobster. (L) Either of these 

 animals may be studied in order to gain a good idea of a crustacean. 

 They are so similar that a description written for study of one will 

 serve as a guide for the other, provided that the student keeps a 

 sharp lookout for little points of difference. The crayfish has some 

 advantages in being smaller, and is usually easier to obtain. 



I. External Structure 



NOTE : In studying the crayfish keep both living and preserved 

 (in alcohol) specimens at hand. The living specimens should be 

 kept in a shallow dish of pure water and examined whenever it is 

 wished to learn the use of any structure seen in the alcoholic speci- 

 mens, which are more convenient to handle. 



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