138 MOLLUSCA. 



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

 called the olfactory organs, but there is no experimental evidence 

 of function. 



12. The attachment of the head and the extent of the man- 

 tle opening around the neck. 



13. The funnel protruding from beneath the mantle on the 

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



14. The median dorsal projection of the mantle. 



\ 15. The tail-fin, its position and shape. What is its function? 

 Draw the animal as seen from the ventral side. 

 Carefully open a specimen by cutting through the mantle 

 a little to one side of the mid-ventral line. 

 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? 



2. The arrangement of the funnel. Why does it have a 

 thin posterior edge? How is it held in position against the 

 mantle. Does it have a valve? Is the funnel movable in the 

 living animal? Is there any provision for movement? 



3. The free edge of the mantle and its relation to the folds 

 beneath the eyes. Do you understand how the water gets into 

 and out of the mantle cavity? 



4. The large retractor muscles of the funnel. How many are 

 there? How can the funnel be pointed in different directions? 

 What need is there for such a provision? 



5. The retractor muscles of the head. How many are there? 

 Are they used in swimming in any way ? " 



6. The rectum, opening near the base of the funnel between 

 two small lateral flaps of tissue. 



7. The ink-bag, dorsal to the rectum and opening into it 

 near the anus. 



8. The gills, extending from a point about midway of the 

 body toward the free edge of the mantle. How many are there ? 

 How are they attached? Why does an animal that is not swim- 

 ming continually pump water through the mantle chamber? 



9. The branchial hearts, at the bases of the gills, rounded, 



