54 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



or other objects (Moore, 1908). Among the small organisms such 

 as hydroids, some species grow toward the source of light just as 

 plants do. With the small fresh-water types, like Spongilla and 

 Plumatella, one finds frequent reference to growth forms, but rarely 

 any details as to their cause and character. Many of the growth 

 forms noted may be ranked as habitat forms, since they are pro- 

 duced by variation in habitat conditions. 



Sessile Single-individual Animals. The forms of these are largely 

 taxonomic, as in the sessile barnacle, gooseneck barnacle, serpulid, 

 oyster, sea mussel, and simple tunicate. The growth forms have been 

 little considered. Pilsbry (1916) has noted long pencil types of sev- 

 eral species of barnacles w'hen crowded (cf. Shelford and Towler, 

 1925). Loeb (1906) has shown that serpulid worms turn toward the 

 light much as do plants, while brachiopods show differences with depth 

 and injury (DuBois, 1916). 



Sedentary Forms. Sedentary animals are those which rarely move, 

 or which give the impression of being stationary because of very slow 

 or only occasional movements. These are usually the passive mem- 

 bers of active groups, and hence the form is essentially taxonomic, 

 with the exception of the coelenterates. The forms are grouped into 

 types with difficulty, but each of the following represents some degree 

 of geometric likeness: (a) hydra, solitary corals, sea anemones; (6) 

 some chitons and limpets; (c) many worms and a few echinoderms, 

 such as Leptosynapta. 



Behavior Forms and Taxonomic Forms. In contrast to the zoo- 

 phytes and common plants, the vast majority of animals are in no 

 sense multiple-individual, sessile or sedentary. Their outstanding 

 feature is activity and the forms concerned bear a corresponding 

 impress. In terrestrial communities motile animals ai^e well-nigh 

 universal; by contrast to sessile organisms, in which response is 

 through growth, their primary adjustment to physical factors is by 

 means of movement. 



The forms of motile animals are usually of direct taxonomic value 

 and at the same time may constitute adaptations determining details 

 of activity. Again, certainly not all the specificities of behavior 

 directly related to structure are of ecological significance. A leaping 

 kangaroo rat may have the same coactions, reactions, and responses 

 to environmental conditions in a grassland community as a ground 

 squirrel which progresses in the more usual way. Accordingly, form 

 in animals is naturally forced into the background. It will therefore 

 not be profitable to go into detail as to form in motile animals. The 

 well-known echinoid, asteroid, snail, bivalve, and vermiform life forms 



