132 MOLLUSCA. 



2. Movements. Any cilia on the foot? Any rhythmic 

 waves passing from end to end? What is the mechanism of 

 foot locomotion? 



3. The movements of the tentacles and proboscis. What do 

 the movements accomplish? 



4. Touch a specimen and see what positions the parts 

 take when it retracts into the shell. If the animal has an 

 operculum see where it is borne, and how it fits into the 

 aperture of the shell. 



BUSYCON (FULGUR, SYCOTYPUS). (Whelk.) 



This large gastropod lives in comparatively shallow water 

 and depends largely on other Mollusca for its food. Examine 

 a retracted specimen and see how the shell is closed by a horny 

 lid, the operculum. Examine expanded specimens in the aqua- 

 ria, and see where the operculum is placed. What position 

 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 many 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 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 Up 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? 



