BRANCH ARTHROPODA 



Arthkop'oda may be characterized as animals having bi- 

 laterally symmetric segmented bodies with jointed appendages 

 and a chitinous exoskeleton. The segments of the body are 

 not so numerous as in the worms. 



This branch includes a vast assemblage of animals which 

 are widely distributed over the earth. They vary in habitat, 

 being aquatic, terrestrial, subterranean, aerial, or some com- 

 bination of these. 



Some are of direct use in furnishing food for man, as the 

 lobster and the bee. Many cross-fertilize plants, and are thus 

 of indirect use to man. As common examples of this branch 

 may be named the lobsters, crabs, crayfishes, spiders, " thou- 

 sand-legs," and insects. 



The digestive system is between the circulatory system and 

 the nervous system. It is not much coiled, but runs almost 

 straight through the body. (See Fig. 09.) 



The circulatory system consists of a dorsal blood-vessel 

 open at the anterior end. The blood is pumped forward. It 

 fills all the irregular spaces of the body, through Avhich it bathes 

 all the tissues and makes its way back to the dorsal vessel. 

 The corpuscles are colorless and ameboid. 



The respiratory system consists of gills in the aquatic forms, 

 and of air-tubes or tracheae in the insects and other terres- 

 trial forms. 



The nervous system consists generally of a double chain of 

 ganglia, connected by a double nerve cord, running along the 

 ventral side of the body. (See Fig. 69, A''.) We should expect 

 to find a pair of ganglia to each segment, but several ganglia rnay 

 be united, as in the crayfish, where there are thirteen well-marked 

 ganglia, the three anterior ones uniting to form the so-called 

 brain. 



Multiplication. — The sexes are usually distinct. Multiplica- 

 tion is generally by fertilized eggs. 



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