178 ARTHROPODA. 



2. Examine the eyes with a lens and see that they are com- 

 pound. 



3. On the lower side^ of the cephalothorax notice the appen- 

 dages. Are they all built on the same plan? Compare them in 

 male and female. Do you know what the modifications are 

 for? Compare the pincers with those of a lobster. The first 

 pair of appendages is called ihe'^chelkerce. Between the bases 

 of the last paii: of walking legs are the chiiana. Behind the 

 chilaria is the broad flat operculum. Does this show evidence of 

 being modified appendages? What is its function? 



4. Between the bases of the cephalothora^cic appendages is 

 the mouth. Do the bases of the appendages show any modifi- 

 cations that may serve as teeth? Can the pincer-bearing ap- 

 pendages be so bent as to be used in feeding? 



5. Along the sides of the abdomen notice the movable spines. 

 How many are there? 



6. Under the operculum are the gills. How many groups 

 are there? Are they arranged in pairs? How are they attached 

 to the body? Are they movable? What reason^ is there for 

 moving them? Examine a bunch of gills, frequently called a 

 gill-hook, and see how it is formed. 



7. At the base of the caudal spine notice the anus. 



Make a drawing of the ventral surface. 



Internal Anatomy. — This shows no very special adaptation 

 and can be pretty well understood by studying a longitudinal 

 section of a small preserved specimen. 



In such a section the following organs may be found : 



1. The dorsal extensor, the ventral flexor, and the Ze^ muscles. 



2. The elongated tubular heart just beneath the dorsal cover- 

 ing, in the posterior end of the cephalothorax and the anterior 

 end of the abdomen. 



3. The alimentary canal, consisting of the esophagus and the 

 anterior and posterior portions of the stomach, which extends 

 posteriorly without much change to the anus. The liver, which 

 surrounds the^^omach and fills the greater portion of the 

 cephalothorax, sends its secretions to the stomach. 



