128 MOLLUSCA. 



9. If you care to see the radula, the organ that especially 

 indicates affinity to the Gastropoda, it can be pulled out by grasp- 

 ing just behind the mouth with pointed forceps and pulling 

 forward. When removed it may be mounted on a slide with 

 water and studied with the microscope. 



Haller: Die Organisation der Chitonen der Adria. Arb. Zool. Inst. Wien., 



4, 1882; 5, 1884. 

 Heath: The Development of Ischnochiton. Zool. Jahrb., 12 (Anat.), 1899. 



GASTROPODA. 



A majority of the Gastropoda have the body protected by a 

 spirally wound shell, and crawl around by means of a flattened 

 muscular foot that forms the ventral portion of the body. 



Examine specimens of Tritia or any other active form 

 and notice: 



1. The relation of the animal to its shell when retracted and 

 when extended. 



2. Movements. Can you determine how are they performed ? 



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). 



This large gastropod lives in comparatively shallow water 

 and depenote 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: 



