GASTROPODA. 131 



finger in its back and prevent it from curling for a minute. It 

 will then generally remain attached to the slide and may be 

 studied from both sides. 



3. How many plates are there? What is the shape of each? 

 Do they apparently join edge to edge or do they overlap? Do 

 the plates extend clear to the margin of the animal? What 

 reason is there for having plates instead of a solid dorsal shell ? 



4. Notice the thickened margin of the animal, and see that 

 dorsally it bears spicules, while ventrally it is smooth and is 

 applied closely to the slide. 



5. Notice the flattened elliptical foot. Do you understand 

 how the animal creeps and adheres? 



6. In front of the foot is the head fold in which the mouth can 

 be seen. 



7. In the furrow bordering the foot are the gills. 



8. Remove the animal from the slide and see how it curls up. 

 Try to unroll it. Explain. 



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. Zcol. 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 such active forms as Alectrion ob- 

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



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

 when extended. 



