ARTHROPOD A 277 



(i) those hurtful to man's interests, (2) those beneficial thereto, 

 and (3) the harmless. State the grounds of your classifica- 

 tion of each form. In what stage of its metamorphosis is 

 each species hurtful or helpful. Extend your own knowledge 

 by inquiry, by observation, and by reading. 



DESCRIPTIVE TEXT 



312. The group of Arthropoda (jointed-legs) embraces more 

 than one-half the species in the animal kingdom, and is cor- 

 respondingly rich in individuals. This is the same as saying 

 that they are remarkably variable and adaptable to various 

 conditions of life. The segmented, bilaterally symmetrical body 

 and the arrangement of the nervous system are the most 

 important points of similarity with the Annulata. The general 

 resemblance is more striking in some of the lower forms (Peri- 

 patus), and in the larval stages of those which undergo a meta- 

 morphosis. The subdivisions of the phylum (if it can be con- 

 sidered a single phylum) are quite diverse and their relation- 

 ships uncertain. There are many parasitic and otherwise 

 degenerate forms which make the problem of classification 

 more difficult. 



313. General Characters. 



1. Elongated, bilaterally symmetrical body. 



2. Segmented; somites heteronomous, and typically grouped 

 into three regions: (i) head, (2) thorax, (3) abdomen. 



3. An outer skeleton, of a secreted chitinous substance. 



4. Each somite has typically a pair of jointed appendages 

 (whence the name arthropod). 



5. Central nervous system similar to that of Annulata: (i) 

 brain, (2) a nerve ring around the esophagus connecting the 

 brain with (3) a ventral, ladder-like chain of ganglia. 



~6. Heart, dorsal to the digestive trace. 



7. Ccelom Tepresented largely by secondary blood spaces 

 connecting with the circulatory system. 



314. General Survey. The symmetry of the Arthropods 

 is very pronounced, except in the case of fixed, parasitic, or 

 otherwise degenerate forms (as barnacles, Sdcculina, etc.). 



