ARTHROPODA 



THIS subdivision of the animal kingdom includes insects, cen- 

 tipedes, spiders, and crustaceans, which together constitute 

 more than half the known species of animals. Although these 

 animals are so unlike in general appearance, it is easy to recog- 

 nize the common characteristics which place them together in 

 one group. The name Arthropoda, meaning " jointed-footed," 

 suggests perhaps one of the most obvious points of resem- 

 blance. The Arthropoda have bilateral symmetry, one side of 

 the body being like the other ; they are covered with a horn-like 

 material (chitin) ; they are divided into segments ; the segments 

 have appendages ; and the appendages are jointed so as to admit 

 freedom of motion. Their manner of growth is peculiar ; they 

 cast off their rigid external coverings and secrete larger coats of 

 mail, and at these periods increase in size or undergo meta- 

 morphosis. 



There are such modifications of these general features as the 

 habits of the different species demand. For instance, the appen- 

 dages may be constructed for walking, swimming, boring, sucking, 

 or the seizure and preparation of food. In some animals the 

 appendages form a part of the breathing-organs, in others are 

 used as organs of sense. Every detail of the organism, down to 

 the hairs, has its special use and function. 



CLASS CRUSTACEA 



The crustaceans vary in size from microscopic minuteness to 

 two feet or more in length. The giant crab of Japan (Kaempferia 

 kaempferi) exceeds this, being commonly from eight to twelve feet 



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