Transport by Air 45 



manently attached to the bottom, allowing food particles to be brought 

 to them by the water. Many of these animals are characterized by 

 radial symmetry; since food may be brought from any direction, a 

 radial arrangement of feeding appendages around the animal's mouth 

 is efficient. Only when locomotion is required is it particularly de- 

 sirable to have a head and a tail end with attendant bilateral sym- 

 metry. Colonial existence is also possible among sessile organisms 

 but would be very clumsy for forms that require active locomotion. 



In aquatic plants and animals the male reproductive cells are carried 

 to the egg cells by water movement, or both are discharged into the 

 surrounding medium where currents bring them together to accom- 

 plish fertilization. The young of sessile organisms are effectively re- 

 moved from the neighborhood of the adults during the larval stage. 

 The planktonic larvae, carried far and wide by the currents, ac- 

 complish the dispersal of the species and the colonization of new areas. 



Many marine and fresh-water animals grow more effectively in a 

 current than in quiet water, and some forms can live only where the 

 water is moving rapidly. Certain caddis fly larvae {Hijdropsyche) 

 living on stream bottoms construct funnel-shaped nets with openings 

 upstearm into which food particles drift. The larva of the mayfly 

 ( Chirotenetes ) braces itself on the bottom with its head upstream and 

 spreads its hairy prothoracic legs like a net with the result that par- 

 ticles of food in the flowing water are funneled into the animal's mouth 

 (Morgan, 1930). The term current demand has been used for the 

 dependence of certain forms, particularly stream forms, on a move- 

 ment of the water, but in some situations the precise reason that a 

 current is necessary is not clear. The larva of the black fly (Sim- 

 tdiitni), for example, will not develop in quiet water even though 

 plenty of oxygen and other obvious needs are available. 



Distribution by Medium 



If the needs are not brought to the organism, the organism must 

 go after them— either by its own locomotion or by hooking a ride on 

 something else. The transportation service of the environment is of 

 particular value to the seeds or larvae of sessile organisms and for 

 those motile organisms that are small or feeble and hence would be 

 very slow in getting around under their own steam. 



Transport by Air. No organism can live permanently floating in 

 the air, but this medium is often useful in the sporadic and intermittent 

 transport of terrestrial organisms ( Wolfenbarger, 1946 ) . The smallest 

 forms, such as the bacteria, can be transported effectively by even a 



