A Guide for the Study of Animals Part 27

Specimens of pond snails, edible snails, and "slugs," and other land snails, and a collection of sh.e.l.ls of various types.

_Definitions._

_Gasteropoda_, the name of the cla.s.s to which the snail belongs.

_Spire_, the coiled portion of the snail sh.e.l.l.

_Aperture_, the opening of the sh.e.l.l.

_Lip_, the edge of the sh.e.l.l forming the margin of the aperture.

_Whorl_, a single coil of the spire.

_Suture_, the depression between the whorls.

_Foot_, the flat disk-like structure on which a snail creeps.

_Breathing pore_, an opening in the mantle used in respiration.

_Lingual ribbon_, the rasp or file like tongue of the snail.

_Observations._

1. Why is a snail called a univalve?

2. Identify the head and mouth of the snail. Watch the snail feeding and examine the mouth of the snail with a lens. What do you notice?

If your aquarium in which the pond snail is living has a green coating (algae) on the side, describe its appearance after the snail has been crawling up and down over it. Explain.

3. How many tentacles has a pond snail? a land snail? Where are the eyes located in each case? What movements of the tentacles do you notice? What is their purpose?

4. How does the rate of locomotion of the snail compare with that of the clam? Find out if the snail can creep backwards or on the surface of the water. Does there seem to be any tendency for the snail to go up and down the sides of the aquaria vertically rather than to the right or left?

5. What does a snail do when disturbed? What is gained by this action?

6. Search for pond snail's eggs on the side of the aquaria. Lift up the bits of cabbage on which the slugs are feeding and search for eggs. Describe what you find in each case, noting the size, appearance, and whether the eggs are laid singly or in ma.s.ses.

7. Find the breathing pore. Describe its position and appearance.

8. Contrast the various types of sh.e.l.ls, and note with care in what respects they differ. Holding the sh.e.l.l with the aperture toward you and the spire pointing up, determine whether each sh.e.l.l has the aperture on the right (right-handed sh.e.l.l) or on the left (left-handed sh.e.l.l). Is the right-handed or the left-handed sh.e.l.l more common?

9. (Optional) By means of some book in the laboratory, determine the scientific name of each of the snails found in the various aquaria in the laboratory.

_Suggested drawings._

a. Drawings to show the pond snail in various positions in the aquarium.

b. A drawing of the slug.

c. At least three different types of snail sh.e.l.l.

_Summary._

1. In what respects does a snail show resemblance to a clam?

2. What are the chief points of difference?

3. What reasons can you suggest for the better development of the sense organs?

4. What advantage has a snail over a clam in the matter of getting food?

5. How does the sh.e.l.l of the snail compare with that of the clam as an organ for protection?

3. THE SQUID--A TYPE OF MOLLUSCA

_To show the Effect of a Much Reduced or Rudimentary Skeleton_

_Materials._

Small squids, and a few large specimens for comparison and dissection.

_Definitions._

_Cephalopoda_, the name of the cla.s.s to which the squid belongs.

_Caudal fin_, a horizontal structure at the posterior end of the squid.

_Chromatoph.o.r.es_, irregular cells in the mantle which give color to the squid.

_Exhalent siphon_, a funnel or tube opening on the ventral side just below the base of the arms or tentacles.

_Pen_, a remnant of an exoskeleton imbedded in the mantle along the dorsal side.

_Ink sac_, a sac containing a dark, sticky liquid which may be thrown out through the funnel into the water. The opening is near the inner opening of the funnel.

_Observations._

1. What is the shape of the squid? To what is this shape adapted?

2. Identify the head and the well-developed eyes.

3. How many arms or tentacles are there? How are they arranged with reference to the mouth? What do you find on the distal ends of the arms? How do the arms vary as to size? What does the position and arrangement of the arms suggest as to their function?