CHAPTER XII 



Arthropoda 



The Arthropoda (Gr. arthron^ a joint; pous^ a foot) include 

 more than one-half the number of species in the animal kingdom 

 and comprise a wide variety of forms, with great significance econom- 

 ically. 



The body, segmented and bilaterally symmetrical as in the 

 Annelida, is covered by a chitinous exoskeleton. The heart is 

 usually elongated and the nerve cord is ventrally situated, while 

 the cerebral ganglia are dorsal and anterior as in the earthworm. 



This Phylum includes insects, barnacles, crabs, crayfishes, 

 spiders, ticks, and scorpions. The bee is an example of a beneficial 

 form; the housefly, of an injurious type. 



Classification 



Class 1. Crustacea. 



Class 2. Onychophora. 



Class 3. Myriapoda. 



Class 4. Insecta. 



Class 5. Arachnid a. 



Characteristics 



1. Marked metamerism. 



2. Appendages jointed. 



3. Body covered by chitinous exoskeleton, secreted by cells beneath 



it. 



4. Bilateral symmetry. 



5. Mouth and anus at opposite ends. ' 



6. Seldom ciliated. 



7. Muscles usually striped. 



8. Dorsal heart, with incomplete circulatory system, the blood 



sinuses extremely important. 



9. Nervous system includes a ventral nerve chain with ganglia and 



paired dorsal cerebral ganglia. 



