ARTHROPODA. 265 



voracious larval stage they devour waste material as 

 scavengers, strip vegetation, spread disease, and furnish 

 food to the higher animals. In both stages they may 

 be parasitic on man and the other animals. 



275. Definition of the Arthropoda. The body is bilater- 

 ally symmetrical and made up of a series of homologous 

 segments. The appendages are typically jointed (whence 

 the name arthropod). The head is made up of three or 

 more fused segments. There is an outer chitinous skele- 

 ton to which lime may be added. The nervous system 

 is in the form of a chain ventral to the digestive tract, ex- 

 cept for a ganglion (brain) above the gullet. Compound 

 eyes are common, but not universal. 



276. Classification and Key to the Phylum. (The following keys 

 are only intended to give the student an idea as to the manner of 

 tracing the classification of animals. For exact work he must refer 

 to the special books on insects.) 



277. A Key to the Classes of Arthropods. 



Chiefly water-breathers; four antennae Class Crustacea. 



Chiefly air-breathers. 



Segments many and similar ; two antennae . . Class Myriapoda . 

 Segments different in different parts and fused into regions. 

 Three body regions (head, thorax, and abdomen) ; two 



antennae; six thoracic legs Class Insecta. 



Two body regions; no antennas; 8 thoracic legs, 



Class Arachntda. 



278. Key to Orders and Common Forms of Myriapoda. 



Not more than one pair of feet to each segment ; antennae long ; 



bodies flattened Order i , Chilopoda. 



Fifteen pairs of legs. 



Eyes in facets (compound) Scutigera 



(1he "skein" centiped). 



Eyes not compound, but numerous Lithobius. 



Twenty-one to twenty-three pairs of legs Scolopendra 



(the poisonous centipedes). 



