CHAPTER XIII 

 PHYLUM ARTHROPODA 



i. INTRODUCTION 



THE ARTHROPODA (Gr. arthron, a joint; pous, a foot) are the 

 crayfishes, water- fleas, barnacles, centipedes, millipedes, scor- 

 pions, spiders, mites, and insects. All of these animals have a 

 common plan of construction, as shown in Figure 199. The body 

 consists of a series of segments some or all of which bear jointed 



FIG. 199. Diagrammatic representation of the structure of an Arthropod. 

 A, eye; D, intestine; F, antenna; G, jointed limbs; //.heart; M, mouth parts; 

 N, nervous system; S, gullet; Sk, chitinous exoskeleton; uS, oS, supra- and 

 infra-oesophageal ganglia. (From Schmeil.) 



appendages (G). The body is covered by a chitinous exoskele- 

 ton (sk) secreted by the cells just beneath it. Within the body 

 is a central tube, the alimentary canal (D), with an anterior 

 mouth opening (at M) and a posterior anal opening. Dorsal 

 to the alimentary canal is a blood-vessel called the heart (H), 

 and ventral to the alimentary canal is the nerve-cord (N). 

 There is a ganglionic mass, the brain (oS), dorsally situated in 

 the head. 



The Phylum ARTHROPODA includes a greater number of species 

 than all of the other phyla of the animal kingdom combined. 



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