CHAPTER XV 

 CRAYFISH, LOBSTERS, CRABS, SHRIMPS, ETC. 



The great branch Arthropoda (Gr. arthron, joint; pous, 

 foot) comprises, as shown by the table of classification on pages 

 55 t 57> fi ye classes; the Crustacea (L. crusta, crust), or 

 crayfishes, crabs, lobsters, barnacles, etc.; the Onychophora 

 (Gr. onyx, claw; phero, bear) or slime slugs; the Myriapoda 

 (Gr. myrios, numberless; pous, foot) or centipedes and thous- 

 and-legged worms; the Arachnida (Gr. arachne, spider) or 

 scorpions, spiders, mites and ticks; and the Insecta 

 (L. insectum, cut into), which in point of number of species, 

 is by far the largest class in the whole animal kingdom. 



The Arthropods get their name from two Greek words mean- 

 ing jointed foot, and the members of the branch are charac- 

 terized by the possession of legs and other laterally arranged 

 paired appendages each composed of several successive jointed 

 parts. The bodies of all Arthropods are bilaterally symmet- 

 rical, and are composed, as are those of the Annelida, or an- 

 nulate worms, of a series of successive segments. They are 

 inclosed by a more or less firm outer cuticle, which not only 

 serves as a protection to the soft body parts within but also 

 provides firm points of attachment for the muscles. This 

 hardened cuticle is called the exoskeleton. 



The internal organs of the Arthropods show a more or less 

 obvious segmentation corresponding with the segmentation 

 of the body wall. The alimentary canal runs longitudinally 

 through the center of the body from mouth to anal opening- 

 The nervous system consists of a brain lying above the esoph- 

 agus and a double nerve-chain running backward from the 

 esophagus, along the median line of the ventral wall, to the pos- 

 terior extremity of the body. This ventral nerve-chain con- 

 sists of a pair of longitudinal- cords and a series of pairs of 



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