CHAPTER VII 

 TYPE V. ARTHROPODA 



The Arthropoda (Gr. apdpov, joint, and Trow, foot) possess 

 more species than any other animal type and include such 

 animals as crabs, scorpions, spiders, centipedes, and insects. 

 They are bilaterally symmetrical, with bodies consisting of a 

 series of segments differing more or less from one another, so 

 that they are said to have heteronomous segmentation, as dis- 

 tinguished from the homonomous segmentation of the Annelida. 

 Typically several segments — in some cases nearly all — are 

 provided with paired appendages attached to the ventral side of 

 the body ; these are the organs of locomotion and always consist 

 of a series of parts placed end to end and connected by movable 

 joints, hence the name jointed-footed applied to the type. The 

 function of these appendages varies greatly. 



They are variously modified so as to be used for running, 

 walking, jumping, swimming, seizing prey, as well as serving for 

 jaws and other mouth parts, as organs of respiration, and in 

 connection with reproduction. The surface of the body is more 

 or less hardened so as to form a sort of external skeleton or 

 shell ; sometimes this is very highly developed, as in the covering 

 of lobsters and crabs, and in other cases the greater portion of 

 the body wall remains relatively soft, as in most spiders, where the 

 chitinous covering is very thin. The sexes are nearly always 

 separate in the Arthropoda ; hermaphroditism occurs in a few 

 cases and also parthenogenesis. The external segmentation of 

 the body shows a certain relationship to the Annelida, and 

 internally this relationship is marked by the structure of the 

 nervous system, which is built on the same plan in the two types. 

 The main subdivisions of the Arthropoda are based upon differ- 

 ences in the method of respiration. 



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