108 MOLLUSCA. 



must the animal assume in the shell to bring the operculum in 

 position? 



Shell. — A somewhat conical tube, spirally wound, somewhat 

 like a spiral stairway. Observe the following parts: 



1. The apex, forming the closed end of the tube. 



2. The spire. How man}^ whorls are there? Do they differ 

 in number in different specimens? In what direction are the 

 whorls wound? (Hold the apex toward you in determining 

 this point.) Examine old and young specim.ens and see if there 

 is evidence that the apex is worn off. 



3. The hody-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 spout-like 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. In structure the shell presents three layers. In a broken 

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

 of the shell; (6) the nacre, smooth and lining the inner surface 

 of shell; (c) the middle layer. How can three layers be secreted 

 by the mantle that lines one of them? 



Draw two figures, one of a perfect and one of a broken shell. 



9. Compare the shell with the shells of other forms, such as 

 Lunatia, Bulla, Haliotus, Crepidula, and Acmsea. 



Soft Parts. — Examine an animal that has been removed from 

 its shell and killed while more or less expanded ^ and see in what 



^ 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 c.c. of turpentine to each 100 c.c. 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 HoU, Mass. 



