552 



INTERRELATIONSHIPS OF ORGANISMS 



surface waters and rely on chance to bring their food organisms into 

 contact with the triggers that discharge poisoned stings. For the smaller 

 jellyfishes the food organisms are members of the great floating assem- 

 blage of minute, rapidly reproducing plankton animals; for the larger 

 coelenterates it is likely to be some of the host of smaller motile food 

 strainers that feed in the subsurface waters. 



Fig. 32.8. Microscopic pond life — a community of interdependent plants and animals. The 

 long cylinders are parts of filamentous green algae, mostly Spirogyra. The large object 

 on the right is one of the trap-bladders of the bladderwort Utricularia, an aquatic flowering 

 plant, with a captured insect larva. In the center a large rotifer sucks out the contents of a 

 Spirogyra filament, cell by cell. Desmids, diatoms, and other small algae are attached to the 

 larger objects, and at the left are stalked food-straining protozoa (Vorticella) — some ex- 

 tended, with beating cilia, and others with stalks contracted like coiled springs and with 

 their cilia pulled in. (Courtesy American Museum of Natural History.) 



Most of these aquatic schemes of food capture have counterparts 

 among terrestrial and aerial animals, but proportionately they are far 

 more important and support a far larger fauna than on land, where the 

 pursuit and capture of individual prey is more generally the rule. 



The medium and reproduction. We have already seen that external 

 fertilization is essentially an aquatic adaptation and that all self-motile 

 gametes are swimmers. We have also seen — in the vertebrates, at least — 

 that the original oviparous habit involved water as a medium to protect 

 the eggs from shock and dessication. 



Terrestrial life has either necessitated a return to the water for repro- 



