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THE AMERICAN NATURALIST [Vol. XLVIII 



not evident because they concern internal, chemical and 

 physical processes which affect neither form nor move- 

 ment. Changes in the enzymes secreted by digestive 

 glands, which accompany changes in food (Jennings, '06, 

 p. 347), are examples. While thus recognizing that re- 

 sponses are concerned primarily with internal processes, 

 we must of necessity refer chiefly to the external phases. 



2. Sessile and Motile Organisms 

 Sessile organisms are those which are sedentary in 

 habit, whether attached or possessing slight powers of 

 locomotion. Motile organisms are those that habitually 

 move about. Vagile or creeping forms as well as swim- 

 ming, walking, flying, burrowing types are included. 

 Most sessile animals are capable of moving their parts, 

 while only a few sessile plants possess this capacity, and 

 these only to a slight degree. 



There is no sharp distinction between sessile (seden- 

 tary) and motile organisms. Every possible gradation 

 exists between fixed non-motile types as trees on the one 

 hand and the pelagic fishes on the other. It is the 

 extremes which we will compare. 



3. The Individual and Its Eelations in Colonies 

 and Groups 



The following comparison of animals and plants is an 

 attempt to distinguish potential or incomplete individuals 

 in colonial organisms and compound organisms which, 

 while not commonly recognized as colonial, are made up of 

 incomplete individuals. 



(a) Animals and Plants made up of Single Individuals 

 The vast majority of animals belong here. Most pro- 

 tozoa, solitary sponges, solitary hydroids, sea anemones, 

 worms not preparing for asexual division, echinoderms, 

 mollusks, arthropods and vertebrates. Only single-celled 

 plants, young seedlings and possibly a few adults of multi- 

 cellular plants which possess but one growing point 



