144 MOLLUSC A 



differ in number in different specimens? In what direction 

 are the whorls wound? (Hold the apex toward you in deter- 

 mining this point.) Examine old and young specimens and 

 see if there is evidence that the apex is worn off. 



3. The body whorl The one that opens to the outside. 



4. The columella. The axis around which the whorls are 

 wound. This is best studied in a broken shell. 



5. The aperture, which is bounded by the inner lip on the 

 columellar side and by the outer lip along the free edge. 



6. The siphonal canal, which forms the spoutlike prolonga- 

 tion of the shell. 



7. The lines of growth. What do they represent? Do 

 they show evidence of injuries that have befallen the shell 

 during the life of the individual? 



8. The three layers of which the shell is composed. In a 

 broken shell notice: (a) the cuticle, worn away from the 

 greater portion of the shell; (b) the nacre, smooth and lining 

 the inner surface of shell; (c) the middle layer. How can 

 three layers be secreted by the mantle? 



Soft Parts. — Examine an animal that has been removed 

 from its shell and killed while more or less expanded 1 and 

 see in what position it was placed in the shell. Compare the 

 number of whorls made by it to the number in the shell. 

 Understand which is right and which is left for the coiled 

 part of the body. Which side was applied to the columella? 

 In determining the position of organs, constantly keep the 

 sides in mind. 



lr This can be accomplished by breaking the shell away with the 

 blade of a hatchet, and when enough of the shell has been removed, 

 loosening the muscle from the columella with the thumb, and then 

 pulling and twisting the animal out. When free from the shell place the 

 animal in sea water to which has been added about one tenth its volume 

 of alcohol and a little turpentine (about 10 cc. of turpentine to each 

 100 cc. of alcohol) and leave for several hours. An animal treated in 

 this way will usually die with its proboscis extended. For the method 

 we are indebted to Mr. Geo. M. Gray, Curator at the Marine Biological 

 Laboratory, Woods Hole, Mass. 



