150 ANIMAL LIFE AND SOCIAL GROWTH 



early stages of the forest evolution on sand dunes. 

 Now the animals and plants of the growing forest 

 community may gain the ascendency over the 

 shifting sand and a stabilized dune is formed which 

 by the action of the wind, may escape from 

 captivity and start moving across country, bury- 

 ing all in its path. 



The organization of such a community reminds 

 one of that of a simple animal such as an Amoeba 

 (Fig. 8). These shapeless lumps of protoplasm, 

 microscopic in size, move about now with one 

 part in advance, now with another. They send 

 out pseudopodia from one side or another which 

 temporarily become the anterior end of the animal. 

 The location of such pseudopodia depends, in 

 large part at least, on the external stimuli that 

 reach them. Many of the animal aggregations 

 exhibit a degree of social organization roughly 

 comparable with the physiological organization of 

 an Amoeba. 



A higher stage in physiological organization 

 is reached when, as in the radially symmetrical 

 jellyfishes, any part of the circumference can move 

 ahead, or the animal can move as a whole in the 

 direction of its aboral pole. Such animals have a 

 set of so-called sense-organs about the margin of 

 the jelly-like disc, each one of which lies at the 

 end of a radius that is similar to any other sense- 



