CHAETOPLEURA, BUSYCON 143 



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 which forms the ventral portion of the 

 body. • 



Examine specimens of such active forms as Alectrion 

 obsoleta, A. trivittata, and Melampus, and notice: 



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

 and when extended. 



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? 



Touch a specimen and see what positions the parts take 

 when it retracts into the shell. If the animal has an oper- 

 culum 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 

 aquaria, and see where the operculum is placed. What posi- 

 tion must the animal assume in the shell to bring the oper- 

 culum in position? 



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

 what 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 



