XVn. THE WORMS, A STUDY OF RELATIONS TO EN- 

 VIRONMENT 



Froblem XXVII. Tlve relation of tlie earthiuorm to its sur- 

 roundings (.optional). {Laboratory Manual, Proh. XXYII.) 



Effect of Surroundings on Plants. — Animals as well as plants 

 are influenced very greatly by their surroundings or environment. 

 We have seen how green plants behave toward the various factors 

 of their environment ; how heat and moisture start germination in 

 a seed ; how the roots grow toward water ; how gravity influences 

 the root and the stem, pulling the root downward and stimulating 

 the stem to grow upward; how the stem grows toward the source of 

 light; and how the leaves put their flat surfaces so as to get as much 

 light as possible; and how oxygen is necessary for life to go on. 



It is quite possible to show that the factors of environment act 

 upon animals as well as plants, although it is much harder to ex- 

 plain why an animal does a certain thing at a certain time. 



How One-celled Animals respond to Stimuli. — We have seen 

 that the single-celled animals respond to certain stimuli in their 

 surrotmdings. The presence of food attracts them ; when they run 

 into an object, they respond immediately by backing away, thus 

 showing that they have a sense of touch. If part of a glass 

 slide containing paramoecia is heated slightly, the animals will 

 respond to the increase in heat by moving toward the cooler end. 

 Many other experiments might be quoted to show that the Hving 

 matter of a simple animal is sensitive to its surroundings. 



The Earthworm in its Relation to its Surroundings. — The 

 earthworm, familiar to most boys as bait, shows us in many ways 

 how a many-celled animal responds to stimuli. Careful observation 

 of the body of a living earthworm shows us that its long tapering 

 body is made up of a large number of rings or segments. The num- 

 ber of these segments will be found to vary in worms of different 

 size, the larger worms having more segments. 



If the two ends of the worm be touched lightly with a small stick 



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