Spatial Relations 327 



In the last chapter we have discussed the more 

 important lines of specialization that have fitted the 

 members of the bottom population to meet or to 

 profit by these conditions. Under the subject "pond 

 societies," further specific illustrations will be cited. 



Life at the surface is less tranquil than on the bottom. 

 There are two kinds of animals that can maintain them- 

 selves there, (i) Those having bodies (together with 

 the air they hold about them) lighter than the w r ater; 

 which rise to the surface like a cork and have to swim 

 in order to go down below. These are mainly adult 

 insects whose problem of getting air we have discussed 

 in the preceding chapter. 



(2) Those having bodies heavier than the water, 

 which maintain themselves at the surface by some sort 

 of hold on the surface film. If free-swimming, they 

 have to swim up to the surface and break through the 

 film before they can use it for support. Certain insect 

 larvae, water-fleas, rotifers, ciliates, etc., are of this 

 habit. Creeping forms must first climb up some 

 emergent stem, break through and then glide away sus- 

 pended underneath the film. Pond-snails and hydras 

 are of this sort. In an aquarium one may see either, 

 hanging suspended, and dimpling the surface where the 

 foot is attached by the downward pull on the film. 



The relations of certain water-fleas to the surface film 

 are particularly interesting. For many of these, such 

 for example as Bosmina, this is a constant source of 

 peril. If in swimming a Bosmina accidentally breaks 

 through this film it falls over on its side and is held there 

 helpless lying on the surface unable to swim away. 

 Unless some disturbance dash it again beneath the 

 water, its only chance for release seems to lie in 

 moulting its skin and slipping out of it into the 

 water. Usually when a catch of surface plancton from 



