NUTRITION AND FEEDING MECHANISMS 203 



occurs in immense numbers, to the exclusion of all other planktonic 

 animals, and smaller swarms of Nyctiphanes are sometimes encountered in 

 the North Atlantic. Along with the permanent plankton occurs a small 

 and variable proportion of larvae of bottom-dwelling forms, especially in 

 shallow waters. Many of the medium-sized planktonic animals are omni- 

 vores; the remainder, forming a small proportion of the community, are 

 carnivores. These are generally of fairly large size, e.g. medusae, cteno- 

 phores, arrow worms, Tomopteris, etc. Adding their depredations to the 

 latter are pelagic fish and various bottom-feeding organisms, particularly 

 in shallow water. At the top of the food chain are carnivorous fish, 

 cephalopods, etc. , which feed on the larger benthic and pelagic animals (39). 

 The population density of zooplankton organisms in a unit volume of 

 sea water provides a measure of their availability as food for other 

 organisms. The average quantity of zooplankton existing below a large 

 unit area of sea is maintained by the phytoplankton in the water below 

 that unit area. The average quantity of organic matter below a unit area 

 of sea surface provides a measure of the amount of food available for 

 pelagic animals in different regions and at different times. Sampling the 

 bottom biomass also reveals the quantity of food available for demersal 

 animals. 



FEEDING MECHANISMS 



Marine animals display most diverse adaptations and specializations for 

 obtaining food; in general, their feeding mechanisms are related to their 

 nutritive habits. Autotrophic, saprozoic and some parasitic animals 

 lack special feeding mechanisms and absorb dissolved substances directly 

 from the medium. In holozoic animals it is possible to recognize definite 

 types of feeding mechanisms determined by common features of the 

 environment and the kinds of food available. Yonge's classification (110) 

 for invertebrates selects three main categories of feeding mechanisms — 



1 . Mechanisms for dealing with small particles. 



2. Mechanisms for dealing with larger particles or masses. 



3. Mechanisms for taking in fluid or soft tissues. 



Each of these categories includes a variety of different devices for dealing 

 with particular kinds of foodstuffs. Many animals make use of a variety of 

 feeding mechanisms, conjointly, or separately as occasion demands (1 17). 



METHODS OF UTILIZING SMALL PARTICLES 



Minute particles, including fine detritus, bacteria and plankton, form the 

 sole nutriment of a great many invertebrates and protochordates. Mechan- 

 isms for collecting fine particulate matter are classified as pseudopodial, 

 ciliary, tentacular, mucoid and setous. The filtering apparatus is usually 

 made more efficient by the secretion of mucus or other viscid matter. 

 Selection of particles is usually quantitative, and there is seldom any 

 mastication of the food. Jorgensen (52) has reviewed certain aspects of 

 filter feeding among invertebrates. 



