4. Identify the exhalent siphon. Where may water enter the mantle cavity? Recalling the action of the siphons in the clam, suggest a method by which a squid is propelled through the water. In what direction must it swim?

5. Split the mantle along the ventral surface and spread apart.

Identify the long plume-like gills, the ink sac, and the inner opening of the exhalent siphon. How many gills do you find?

_Suggested drawings._

a. The squid side view.

b. The squid from the ventral side with the mantle split open, arrows to show direction of water.

_Summary._

1. In what ways does a squid show relationship to the clam and the snail?

2. What has a squid gained through the reduction of its exoskeleton?

What has it lost? What changes were necessary in its structure to offset the loss of an exoskeleton?

4. A COMPARATIVE STUDY OF MOLLUSCA

_Materials._

Specimens of as many different kinds of mollusks as possible, charts, books.

_Observations._

1. What is the symmetry?[3]

[3] This and the following questions are to be answered for each specimen. The answers may be tabulated if desired.

2. Is the body segmented or unsegmented?

3. Are lateral appendages present or wanting?

4. Is an exoskeleton present or wanting? If present, is it univalve or bivalve; if absent, what other means of protection has been developed to take its place?

5. Is the animal fixed, or is it free to move? If fixed, in what way?

If it moves, what is the method and organ of locomotion?

6. What are the organs of respiration? What is their character?

7. How is food obtained?

8. What senses are probably present? What sense organs are present?

9. What is the habitat?

10. In what ways if any does the animal show degeneration?

_Summary._

1. What characters are common to all mollusks?

2. What is the princ.i.p.al means of protection among mollusks?

3. Name three causes of degeneration among mollusks.

5. MOLLUSCA: REVIEW AND LIBRARY EXERCISE

_Characteristics._

1. What are the general characteristics of mollusks?

2. Name the princ.i.p.al cla.s.ses and give the characteristics of each.

_Morphology._

3. What is peculiar about the structure of a clam's heart? What is its position? Contrast with the heart of a crayfish.

4. Make cross-sectional diagrams to show the arrangement of parts in a clam: (a) in the region of the umbone; (b) in the region just in front of the posterior muscle; (c) in the region of the anterior muscle.

5. Describe the various types of eyes found in mollusks, and their location.

6. Describe the tongue or lingual ribbon of the snail, and its use.

7. What is the operculum of snails? its use?

_Physiology._

8. Describe the circulation of water through the siphons and mantle cavity of a clam. How is it caused? What three uses has it?

9. What are the princ.i.p.al facts about the development of fresh-water clams?

10. Describe the circulation of blood in a clam.

11. What various methods of locomotion are found among mollusca?

_Economics._