=Summary of the Biological Process.=—An earthworm is _a living machine which does work_ (digging and crawling; seizing, swallowing, and digesting food; pumping blood; growing and reproducing). To do the work it must have a continual _supply of energy_. The energy for its work is set free by the protoplasm (in its microscopic cells) undergoing a destructive chemical change (_oxidation_). The waste products from the breaking down of the protoplasm must be continually removed (_excretion_). The broken-down protoplasm must be continually replaced if life is to continue (the income must exceed the outgo if the animal is still growing). The microscopic cells construct more protoplasm out of food and oxygen (_assimilation_) supplied them by the processes of nutrition (eating, digesting, breathing, circulating). This protoplasm in turn oxidizes and releases more energy to do work, and thus the cycle of life proceeds. CHAPTER VII CRUSTACEANS CRAWFISH SUGGESTIONS.—In regions where crayfish are not found, a live crab may be used. Locomotion and behaviour may be studied by providing a tub of water, or better, a large glass jar such as a broad candy jar. For suggestions on study of internal structure, see p. 58. =Habitat.=—Do you often see crawfish, or crayfish, moving about, even in water where they are known to be abundant? What does your answer suggest as to the time when they are probably most active? Why do you never see one building its chimney, even where crayfish holes are abundant? Is the chimney always of the same colour as the surface soil? Are the crayfish holes only of use for protection? In what kind of spots are crayfish always dug; Why? What becomes of crayfish when the pond or the creek dries up? How deep are the holes? How large are the lumps of mud of which the chimney is built? How does it get them out of the hole? Why is the mud built into a chimney instead of thrown away? (What would happen to a well with its mouth no higher than the ground?) Why are crayfish scarce in rocky regions? How does the colour of the crayfish compare with its surroundings? Is its colour suited to live in clear or muddy water? Define protective colouration.