ADAPTATIONS 151 



However, a further example of the purposive 

 actions of these minute forms may be given 

 before we leave them. There is a foraminifer 

 named Arcella which makes for itself a round Arcella 

 concavo-convex test. On the concave side 

 there is a single central round opening 

 through which it is able to protrude its 

 pseudopodia. 



One may colloquially speak of this surface 

 as its front and the convex aspect as its back. 

 Now sometimes it will occur that Arcella will 

 tumble over on to its convex back and in that 

 position one would suppose that it would be 

 as powerless and even more helpless than the 

 beetle which one sees feebly moving its legs 

 and wondering apparently how it is ever going 

 to get upon them again. 



Arcella has its own method of achieving this 

 task. It sets to work and by means of some 

 mechanism of which we know nothing, it 

 produces gas-bubbles in its interior. 



These are arranged in one of two ways. In 

 some cases they form at one side of the body 

 only. The result is that that side is floated 

 upwards and the little creature comes to stand 

 on its edge. From that position it is easy, by 

 the aid of its pseudopodia, for it to resume 

 its normal position with the pseudopodia 

 downwards. 



