CHAPTER IX 



STRUCTURAL EFFECTS PRODUCED 

 BY AGGREGATIONS 



A HE structure of animals 



is relatively hard to change. Environmental 

 effects are more likely to be shown by animals as 

 changes hi behavior or hi other physiological 

 processes than as changes hi bodily structure. 

 In plants, on the other hand, such modifications 

 readily appear as structural changes; for example, 

 the growth-form of trees and shrubs depends in 

 part on whether they grow hi open or in crowded 

 conditions. With sessile animals, similar growth- 

 form changes occur as a result of crowding. The 

 sea-mussel, Mytilus, has one shell-form if it grows 

 hi an isolated region and an entirely different one 

 if it is crowded. Similarly many other marine 

 annuals, barnacles, ascidians, sea anemones and 

 sponges are more slender and elongated if they grow 

 in dense clusters than if they grow relatively alone. 

 Two of the most fundamental properties of 

 living things are then* organization along an axis 

 and their sex. Not all organisms are sexual al- 

 though most of them are, but almost all show some 



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