5. Which somite of the crayfish is without appendages? How many somites are there in a crayfish's body, if each somite bears only one pair of appendages, as many scientists believe? How many of these are in the head; thorax; abdomen? 6. Compare the nervous system of the crayfish with that of the earthworm as regards efficiency. Upon what do you base your answer? 7. Name two points in which earthworms and crayfishes are alike. Name three in which they differ. 8. How are crayfishes caught for market? Where do those sold in Chicago usually come from? How are they shipped? 9. Compare the young forms of a crayfish and a crab. 10. Describe any five different crustacea. 11. Describe the work done by the United States government and by the state governments to protect and to perpetuate the lobster. Why is it thought necessary to do this? 12. Discuss the process and the advantages and disadvantages of molting, as seen in the crustacea. 13. Name two advantages in having such a shell as crustacea have. Name two disadvantages. On the whole, is such a shell favorable to an animal's chances of success or is it not? 14. Give the curious myth about goose barnacles. 15. What crustacea are parasitic? Give an account of one. 16. Why are barnacles classed among crustacea? Where were they once classed? Why may they be considered degenerate, even though not parasitic? How do they manage to succeed? What is their economic importance? How are their effects checked or prevented? 17. Describe some of the odd means of self-protection shown among crustacea. 18. Describe a compound eye. Give two theories as to what can be seen with a compound eye. Why do we not know, instead of theorizing? 19. What is the economic value of the very small crustacea? 20. Discuss the value to man of the various forms of crustacea. CHAPTER V ADAPTATIONS FOR PROTECTION FROM ENEMIES #A. THE EXOSKELETON# 1. THE CLAM--A TYPE OF MOLLUSCA _To Show the Effect of a Heavy Exoskeleton_ _Materials._ Living clams in aquaria, with enough moist sand to cover the clams, preserved clams, sets of matched clamshells, a few shells with the hinge unbroken, evaporating dishes, hydrochloric acid. _Definitions._ _Mollusca_, a branch of the animal kingdom including those animals with soft, unsegmented bodies, inclosed in two folds of skin known as the mantle. They are often called shellfish as most of the forms have a shell. _Lamellibranchiata_ or _Pelecypoda_, names given to the class of Mollusca to which the clam belongs. The former term refers to the broad, flap-like gills and the latter to the hatchet-like foot. _Valve_, one of two parts of the clamshell. _Hinge ligament_, the elastic structure which fastens the valves together at the dorsal margin. _Umbones_, a pair of elevations near the anterior end of the shell. _Lines of growth_, concentric lines around the umbones.