THE PHYSICAL ENVIRONMENT OF ORGANISMS 



551 



in the methods employed to secure it. Aside from the associated prob- 

 lems of locomotion, one of the main differences between the water and 

 the land is the relatively huge amount of food material that is dispersed 

 in the upper layers of nearly all aquatic situations. This food is chiefly 

 in the form of small living plants and animals, together with a considerable 

 quantity of nonliving food — dead organisms or fragments of them. The 

 minute drifting plant and animal life is collectively called the plankton. 

 Existence of this abundant and constant food supply has permitted the 

 development of an important aquatic fauna that feeds upon it. The 

 members of the fauna include: 



Motile Food Strainers. Among these are many fishes, some of the 

 whales (Fig. 32.4), and a number of 

 both small and medium-sized in- 

 vertebrates. All are equipped with 

 some type of sieve (frayed whale- 

 bone, gill rakers, or mouth or foot 

 bristles), through which quantities 

 of water are strained while the ani- 

 mals swim through the suspended 

 food particles. These animals do not 

 pursue individual prey but simply 

 strain a sufficient quantity of water 

 to yield the requisite food. 



Nonmotile Food Strainers of Quiet 

 Waters. These are sedentary forms, 

 such as the sponges (Figs. 13.4 and 

 B.3), oysters and clams (Fig. 32.5), 

 protochordates (Figs. 27.5 and B.26) 

 and others, that by means of cilia 

 or flagella cause a slow, steady stream of water to flow through body 

 passages that are equipped to retain the food particles carried in with the 

 water current. Most of these organisms utilize the same water currents 

 to bathe their respiratory surfaces and so combine breathing with food 

 gathering. 



Nonmotile Food Strainers of Flowing Waters. This is a smaller group 

 very characteristic of riffles and gentle rapids in brooks and rivers. It is 

 largely composed of aquatic insect larvae that either spin silken nets 

 (Fig. 32.7) or, clinging to some stable support, hold sievelike body parts 

 (bristly fringed forelegs or mouth parts) out into the current and wait 

 for food particles that are brought to them. 



Food Trappers or Stingers. The most conspicuous of these are the 

 jellyfishes and other coelenterates, which, floating near the surface, trail 

 tentacles armed with stinging cells through the densely populated sub- 



Fig. 32.7. A food strainer of swift streams. 

 The curved, trumpetlike silken funnel of 

 the caddis worm Neureclipsis, attached to 

 a rock in a mountain brook in Pennsyl- 

 vania. The larva lives in the narrow end 

 of the funnel and feeds upon the food par- 

 ticles strained from the flowing water. 

 (Photo by Dr. Leonard N. Allison.) 



