GENERAL PHYSIOLOGY 177 



special receptive regions, by endosmosis. This type of food-getting 

 may be regarded as a degeneration or adaptation of the holozoic 

 method, the speciaHzed absorptive areas being reminiscent of former 

 mouths, while the pathogenic effects of some types of parasites are 

 interpreted as due to the secretion by the parasite of digestive 

 fluids which cause cytolysis of the host cells. Holophytic or auto- 

 trophic nutrition, characteristic of the green plants, is quite differ- 

 ent in principle from the other two. Digestive processes typical 

 of the majority of animals, as well as the intake of solid or dissolved 

 food, are absent. A highly labile substance, chlorophyll, is manu- 

 factured in the presence of light and usually by specialized plastids— 

 chromoplastids— of the cell. Chlorophyll is very sensitive to light 

 and in some way not yet understood is instrumental in utilizing the 

 radiant energy of the sun to form complex, energy-holding com- 

 pounds. Plants thus become the great banking house for animals 

 and their capital is the apparently inexhaustible energy of the sun. 

 Only those Protozoa with chlorophyll, standing on the boundary line 

 between plants and animals, have this power to directly untilize the 

 sun's energy (see infra p. 197). Heterotrophic nutrition, finally, is 

 characteristic of those Protozoa which combine any two of the above 

 methods of acquiring raw materials. Some forms combine holozoic 

 with saprozoic methods; others holozoic and holophytic; others 

 sap^oph^i;ic and holophj'tic, and some combine all tlu-ee. 



(a) Holozoic Nvtritioii.— The great majority of Protozoa are 

 holozoic in their methods of food-getting, and we may distinguish 

 two main groups, the continuous feeders, and the occasional feeders. 

 Continuous feeders are those forms with permanently open mouths 

 through which a constant current of water is maintained by action 

 of the peristomial motile apparatus (see p. 147). Minute forms of 

 life, especially Bacteria, are carried by these currents into the endo- 

 plasm where they undergo digestion in improvised stomachs or 

 gastric vacuoles (see p. 187). The majority of ciliates belong in 

 this group including many of the holotrichous and all of the hypo- 

 trichous, heterotrichous and peritrichous ciliates. 



The occasional feeders, like carnivorous t^^pes of Metazoa, feed 

 whenever chance brings prey within the radius of their activity, 

 and many of them, like cannibals, are guilty of feeding at times upon 

 their close relatives (Maupas, 1SS3, Joukowsky 1S98, Dawson 

 1919, Lapage 1922). In some cases balloon-like membranes are 

 unfolded and spread out like sails for the direction of food currents 

 to the mouth as in Pleuronema chrysalis. Such forms are interme- 

 diate between the constant and occasional feeding types. In other 

 cases great net-like traps are spread for the capture of unwary 

 diatoms, desmids or smaller Protozoa, as in the Foraminifera (Fig. 

 87). In other cases the microscopic hunters, like men in shooting 

 boxes, lie in wait for their prey. Here long tentacles usually 

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