STRUCTURE AND FUNCTION 63 



leaves which were scattered where the worms could reach 

 them. Have the worms moved them about at all ? If so, 

 where are the leaves left ? Have any been eaten, in part 

 or entirely ? If so, is there any evidence of selection, either 

 as to the kind of leaf or the portion of leaf eaten ? If earth- 

 worms select food, what senses would be useful for the pur- 

 pose ? Have you any evidence that earthworms possess 

 such senses ? 



4. Looking through the dorsal wall, notice the meander- 

 ing red line, seen more easily in some regions than in others. 

 This is the dorsal blood vessel. How long is it? Where 

 is it wider.' Where narrower .'' Notice its pulsations. 

 How many times per minute does it pulsate.' In which 

 direction is the blood forced } Is there a corresponding ven- 

 tral blood vessel? Place a small worm between two pieces 

 of glass, so that you may see through it more easily, and 

 identify the blood vessels encircling the digestive canal, 

 near the anterior end. These are the so-called " hearts " 

 of the earthworm. If possible, decide in which direction 

 the blood flows through them. 



5. The food canal, or alimentary canal, lies underneath 

 the dorsal blood vessel, and is usually easily seen, especially 

 if it is full of food. Notice it when the worm is fully 

 stretched and again when it is contracted. How long is 

 the canal ? Why does it wrinkle when the worm contracts ? 

 Where does it open to the outside ? Why does it need to ? 



6. Where do you infer respiration must take place in this 

 animal ? Why do you think so ? What fits this surface for 

 such a purpose ? Why does an earthworm seem so un- 

 comfortable when it is too dry ? 



7. Where do earthworms live ? What conditions are necessary in 

 their habitat ? 



8. When do earthworms usually leave their burrows ? Why at that 



