PHYLUM ARTHROPOD A 275 



This number is estimated at from one million up, although only 

 about four hundred thousand species have been described. 



Economically certain members of this phylum are of great im- 

 portance. We need only mention the lobster as an article of 

 food, the honey-bee as a producer of honey and beeswax, the silk- 

 worm as the source of silk, the gypsy-moth caterpillar as a de- 

 stroyer of trees, and the mosquito and housefly as carriers of 

 disease germs. 



The ARTHROPODA may be grouped for convenience in the fol- 

 lowing manner : - 



Phylum Arthropoda. CRAYFISH, CRABS, CENTIPEDES, IN- 

 SECTS, SPIDERS, SCORPIONS, TICKS. Triploblastic, bilaterally 

 symmetrical animals; anus present; ccelom poorly developed; 

 segmented, somites usually more or less dissimilar; paired, 

 jointed appendages present on all or a part of the somites; 

 chitinous exoskeleton. 



SECTION A. BRANCHIATA. Mostly aquatic ARTHROPODA 

 usually breathing by means of gills. 



Class I. Crustacea. Examples: crayfish (Fig. 202), water- 

 flea (Fig. 21 1), barnacle (Fig. 214), sow-bug (Fig. 220). 



SECTION B. TRACHEATA. Air-breathing ARTHROPODA with 

 tracheae (Fig. 243). 



Division i. Protracheata. Primitive tracheates which pos- 

 sess nephridia and other annelid characteristics, and tracheae and 

 other insect characteristics. 



Class II. Onychophora. Example: Peripaius (Fig. 228). 



Division 2. Antennata. Tracheates with one pair of an- 

 tennae (Fig. 250). 



Class III. Myriapoda. Antennata with many similar legs. 

 Examples: centipedes (Fig. 233), millipedes (Fig. 232). 



Class IV. Insecta. Antennata with three pairs of legs, and 

 usually wings. Examples: grasshopper (Fig. 249), honey-bee 

 (Fig. 236). 



Division j. Arachnid a. Tracheates without antennae, and 

 with tracheae, book lungs, or book gills. 



