74 Laboratory Guide in Zoology 



THE CLASSIFICATION OF THE ARTHROPODA 



The branch Arthropoda which includes the jointed-footed 

 animals, is one of the largest and most important zoolog- 

 ical divisions. Its members differ from the Annulata in 

 having jointed legs or other appendages attached to the 

 body segments, and from the Mollusca in having the body 

 definitely divided into segments. They have an exoskel- 

 eton composed of chitin or of chitin and calcium combined. 

 They possess a specialized nervous system, composed of a 

 dorsal brain and a double row of ganglia along the ventral 

 side of the body cavity. In general, the body is bilaterally 

 symmetrical. 



The following classes of Arthropoda are now recognized 

 by leading authorities : — 



Class I. Crustacea. Lobsters, Crabs, Shrimps, and 

 Barnacles. 



Class II. Onychophora. Peripatus. 



Class III. Myriapoda. Centipedes and "Thousand-legs." 



Class IV. Insecta or Hexapoda. Bees, Beetles, Butter- 

 flies, and other insects. 



Class V. Arachnida. Spiders, Scorpions, and Mites. 



The first class, the Crustacea, includes the smallest as 

 well as the largest forms of the Arthropoda ; forms which 

 vary in size from the microscopic Cyclops to the huge 

 crabs, some of which are of enormous size. The typical 

 crustacean is made up of twenty-one segments, divided 

 between the three great divisions of the body, — head, 

 thorax, and abdomen. The segments of the head and 

 thorax have become united, forming the cephalothorax, 

 while the individual segments of the great divisions of the 

 body have become so fused together that it is difficult 

 to distinguish the typical number. There are two pairs 



