204 THE BIOLOGY OF MARINE ANIMALS 



Pseudopodial Mechanisms 



Feeding by the use of pseudopodial processes is characteristic of 

 rhizopods. In amoebae food is captured by pseudopodia which extend 

 around the food particle and engulf it, together with a certain amount of 

 water. The pseudopodia of foraminifers (Polythalamia) are long-branching 

 and anastomosing filaments. The protoplasm in each filament shows 

 active streaming movements, one stream being directed centrifugally, the 

 other centripetally, and the actual length of a pseudopod is determined by 

 the relative magnitude of the two currents. Food particles encountering 

 the pseudopods adhere to their sticky surfaces, and are either carried directly 

 towards the body with the return stream, or out to the tip and thence back 

 to the body where they are ingested. Feeding is similar in radiolarians, 

 which also entrap food particles on sticky pseudopodia and carry them into 

 the body by protoplasmic streaming movements (Fig. 14.18, p. 612). 



Feeding in these groups is largely a passive affair, and depends on chance 

 encounters with food particles. Radiolarians are predominantly floating 

 forms which feed upon fine planktonic materials. The Foraminifera include 

 both pelagic and benthic forms which feed upon fine plankton. 



Among colourless flagellates, some are saprophytic, others holozoic in 

 feeding habits. Holozoic species are frequently amoeboid in habit, and 

 ingest their prey by means of pseudopodia. More specialized is the feeding 

 of Noctiluca, which catches small planktonic organisms on a sticky fila- 

 ment. The food particles are aggregated into balls or strands, and conveyed 

 to the cytostome where they are ingested (35, 46). 



Ciliary Mechanisms 



Ciliary methods of feeding are widespread, especially in sedentary 

 species, and are encountered in various guises in many phyla. Cilia and 

 flagella which primitively possessed a locomotory function, have in these 

 forms become concerned with food-getting. Their activity sets up water 

 currents which carry food particles to the animal, and they are further- 

 more arranged in definite tracts by which captured material is sorted and 

 transported either towards the mouth, if suitable, or to the region where 

 ejection takes place. Frequently, a viscid mucous sheet is secreted for 

 trapping the particles. Many ciliary feeders appear to exercise mechanical 

 rather than chemical selection of food particles, selecting or rejecting on 

 the basis of particle-size. 



Protozoa. Tn ciliates (Infusoria) the cilia are frequently organized in 

 complex patterns. Some are mouthless endoparasites (Astomata), in- 

 habiting the gut of invertebrates; some of the holotrichs are raptorial. 

 Many species, however, produce a feeding current by means of complex 

 ciliary apparatuses in the cytopharynx, which is a funnel extending 

 into the endoplasm. Food particles which gather in the bottom of the 

 pharynx become incorporated in food vacuoles and are engulfed by the 

 endoplasm. 



