278 ZOOLOGY 



Silk-worm. The various stages may be studied in con- 

 finement. 

 VIII. Group the Arthropoda known to you, in three classes: 

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

 and (3) those having no apparent relation to man. State the 

 grounds of your classification 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 whole animal kingdom, and is 

 correspondingly rich in individuals. This is the same assajdng 

 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. 



313. General Characters. 



1 . Elongated, bilaterally symmetrical body. 



2. Segmented; somites usually 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 typically with 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 gan'glia. 



6. Heart, dorsal to the digestive tract. 



7. Coelom greatly reduced and not forming a part of the 

 rather spacious secondary body cavity (hsemocoele). 



314. General Survey. — The symmetry of the Arthropods 

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

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



