40 PRACTICAL ZOOLOGY. 



the insects studied, (2) the characters of spiders, (3) the 

 characters of myriapods, (4) the characters of crustaceans, 

 and by comparing these four lists make the following : 



1. A list of characters which they all have in common. 



2. A list of characters which insects have, but which the 

 others have not. 



3. A list of characters which spiders have, but which 

 the others do not have. 



4. A list of characters which myriapods have, but which 

 the others do not have. 



5. A list of characters which crustaceans have, but which 

 the others do not have. 



Which are more striking, the differences or the resem- 

 blances, in thus comparing these four groups? 



The four classes, Insecta, Arachnida, Myriapoda, and 

 Crustacea, constitute the branch of the animal kingdom 

 called the Arthropoda. 



List 1 ought to be an approximate definition of the 

 branch Arthropoda. 



THE EARTHWORM. 



Place a live earthworm on a newspaper on the table, 

 and watch its motions. How does it crawl ? Is the same 

 end always foremost ? Are the two ends alike ? Near 

 the anterior end is the thick white girdle; is it a complete 

 girdle ? Count the segments ; are they all alike ? Com- 

 pare the upper, dorsal, surface with the under, or ventral, 

 surface. A dorsal vessel can usually be seen under the 

 dorsal surface ; watch for pulsations. With the forceps 



