Feeding and Digestion 145 



ments. There is good evidence that in these higher animals the absorption 

 occurs by way of the gut, into which the material must first pass. It has been 

 generally held that fresh-water fishes and amphibians do not drink significant 

 quantities of water, but the ability of these animals to absorb dissolved sub- 

 stances by way of the gut renders this questionable, as do recent observations 

 on the concentration of colloidal material in the digestive tract of the gold- 

 fish. 2 



Particulate Food. For the vast majority of animals, other organisms and 

 particulate organic detritus constitute the principal sources of food. The best 

 classification of the innumerable mechanisms used by animals in dealing with 

 such foods is that of Yonge, ^'^' who separates feeding mechanisms into three 

 large categories: (1) mechanisms for dealing with small particles; (2) mech- 

 anisms for dealing with large particles or masses; (3) mechanisms for taking 

 in fluids or soft tissues. 



Mechmtisnis for Dealing with Small Particles. Such mechanisms are found 

 only in aquatic species; most multicellular animals possessing these mechanisms 

 are sessile or quite sluggish. A viscous secretion is usually produced, and in 

 this the particles are trapped prior to ingestion. Yonge subdivides these mech- 

 anisms into six groups: 



PSEUDOPODIAL. Thcsc are exemplified by the reticulate pseudopodia of the 

 radiolarians and foraminiferans, upon which particles are caught and swept 

 inward toward the main portion of the body for ingestion. 



CILIARY. These are very widespread among animals. Special patterns of 

 distribution and activity of cilia are virtually as numerous as the number of 

 species which possess such mechanisms. Generally speaking, feeding currents 

 are produced which bring large volumes of the medium into close association 

 with the feeding parts; the food particles are carried along definite tracts where 

 they are entangled in a secreted viscid material and finally are carried into 

 the mouth. Most fine-particle feeders appear to show fittle or no chemical but 

 only mechanical selection of food particles, accepting or eliminating the par- 

 ticles on the basis of size; ciliate Protozoa may show chemical selection. Cihary 

 mechanisms for fine-particle feeding are found among the ciliate protozoans, 

 sponges, many tubicolous annelids, echiuroids, rotifers, entoprocts, ectoprocts, 

 phoronids, brachiopods, some gastropods, most lamellibranchs, and many 

 tunicates and cephalochords. In some of these the mucous filter is not used. 



TENTACULAR. Somc holothuroids possess tentacles about the mouth on 

 which small particles fall and then become entrapped in mucus. The tentacles 

 are then pushed into the mouth and the material removed. 



MUCOID. The sessile gastropod Vermetus extrudes from its mouth a film of 

 mucus which is retracted at intervals, bringing into the mouth the particles 

 entrapped upon it. 



MUSCULAR. The scyphozoan jellyfish has, instead of a single mouth, 

 numerous fine pores leading to the coelenteron. Particle-laden water is drawn 

 in and the water is expelled by rhythmic muscular contractions of the bell. 

 The particles are filtered out in the digestive cavity. The polychaete Chaetop- 

 terus and the echiuroid Urechis secrete mucous bags which filter out food 

 particles and are eventually swallowed. Currents which fill the bags are set 

 up by parapodial movement in Chaetopterus and by body peristalsis in 

 Urechis. ^^^ By comparing the filtration of different proteins MacGinitie 



