274 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) the harmless. State the grounds of your 

 classification of each form. In what stage of its metamor- 

 phosis is each species hurtful or helpful. Extend your own 

 knowledge by inquiry, by observation, and by reading. 



DESCRIPTIVE TEXT. 



305. The group of Arthropoda embraces more than one- 

 half the species in the animal kingdom, and is correspondingly 

 rich in individuals. The segmented, bilaterally symmetrical 

 body and the arrangement of the nervous system are the most 

 important points of similarity with the Annulata. The gen- 

 eral resemblance is more striking in some of the lower forms 

 (Peripatus), and in the larval stages of those which undergo 

 a metamorphosis. The subdivisions of the phylum (if it can 

 be considered a single phylum) are quite diverse and their 

 relationships uncertain. There are many parasitic and other- 

 wise degenerate forms which make the problem of classifica- 

 tion more difficult. 



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 oesophagus connecting the 

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



6. Heart, dorsal to the digestive trace. 



7. Coelom represented largely by secondary blood spaces 

 connecting with the circulatory system. 



