CHAPTER III 

 FORMATION OF ANIMAL AGGREGATIONS 



The method of formation of animal aggregations differs with the 

 degree of integration and with the different types of integrating 

 factors. The discussion to be given here is not necessarily exhaustive, 

 but the examples included may serve to illustrate the common meth- 

 ods and some of the problems involved. 



One whole group of aggregations of individuals that are ordinarily 

 solitary is caused by tropistic responses to environmental stimuli. 

 Deegener recognizes one phase of this type of aggregation in his 

 grouping called "symphotium" which occurs when individuals col- 

 lect about a source of light. Aggregations of this general type may 

 be called "syntropia," as suggested earlier. The method of forma- 

 tion of such aggregations attracted much attention in the three dec- 

 ades and a half of J. Loeb's work in this field, from about 1888 to 

 1923. Loeb and his immediate followers were concerned chiefly with 

 aggregations which result from environmentally forced orientations 

 and movements. 



FORCED MOVEMENTS 



When exposed to certain stimuli, some animals react as if they 

 were automatons forced by the interaction between their own organi- 

 zation and their environment to move in a certain direction and to 

 aggregate when available space is limited. The term "tropism" was 

 at one time reserved for such reactions. These are well illustrated by 

 the response of the larvae of the annelid worm Arenicola to light. 



These worms burrow as adults in the sandy tidal flats of the 

 Atlantic Coast south of Cape Cod. The eggs are deposited in large 

 numbers in a jelly-like mass which is attached at the opening of the 

 burrow. The eggs develop into free-swimming ciliated larvae having 

 two eye-spots symmetrically placed near the anterior end. Immedi- 

 ately after hatching, the larvae are strongly positive to light and 

 negative to gravity. Accordingly, they travel to the surface of the 



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