116 MOLLUSC A. 



3. In what direction it can swim best. Can it swim in the 

 other direction? How does it swim? Is the fmmel movable? 

 How does it guide its movements ? 



4. Its color. Irritate it and see what happens. What pur- 

 pose does the change in color serve? What is the ink for? 



5. What happens when the end of a finger is placed within 

 the circlet of tentacles of an animal about two inches long that 

 is being held firmly? 



Using an adult specimen, observe: 



6. The arrangement of the arms on the head. Are they ar- 

 ranged in any definite order? Are they all alike? 



7. The suckers of the arms. Do they follow the same order 

 on all of the arms? 



8. The structure of a sucker. Notice the peduncle, outer 

 thin margin, horny ring, and piston. Is the horny ring smooth? 

 W^hat is its function? How does the sucker work? Split one 

 and draw the cut surface. 



9. The mouth. Where is it placed? Notice the tips of the 

 horny beak. Which jaw is longest? 



10. The eyes. 



11. The fold of tissue behind each eye. These have been 

 called the olfactory organs. 



12. The attachment of the head and the extent of the mantle 

 opening around the neck. 



13. The funnel protruding from beneath the mantle on the ven- 

 tral surface. Notice the position and character of its opening. 



14. The median dorsal projection of the mantle. 



15. The tail-fin, its position, shape, and color. What is its 

 function? 



Draw the animal as seen from the ventral side. 



Carefully open your specimen b}^ cutting through the mantle 

 a little to one side of the mid-ventral line, and expose the man- 

 tle chamber. 



Notice : 



1. The thickness and character of the mantle and its relation 

 to the rest of the body. Why does it need to be so muscular? 



