GENERAL PHYSIOLOGY 185 



of animals, as well as the intake of solid or dissolved food, are 

 absent. A highly labile substance, chlorophyll, is manufactured 

 in the presence of light and usually by specialized plastids— chromo- 

 plastids— 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. 

 Heterotrophic nutrition, finally, is characteristic of those Protozoa 

 which combine any two of the above methods of acquiring raw 

 materials. 



The great majority of Protozoa are holozoic in their methods of 

 food-getting, and w T e may distinguish two main groups, the con- 

 tinuous 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 peri- 

 stomial motile apparatus (see p. 164). Minute forms of life, espe- 

 cially Bacteria, are carried by these currents into the endoplasm 

 where they undergo digestion in improvised stomachs or gastric 

 vacuoles (see p. 193). Chejfec (1929) estimates that Paramecium 

 caudatum may thus ingest and digest from two to five million 

 Bacterium coll in twenty-four hours. The majority of ciliates, 

 including many of the holotrichous, hypotrichous, heterotrichous 

 and peritrichous ciliates, belong in this group. 



The occasional feeders, like carnivorous types 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, 1883, Joukowsky, 1898, 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 (Fig. 199, p. 482). Such 

 forms are intermediate 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 Foramin- 

 ifera (Fig. 10, p. 32). In other cases the microscopic hunters, like 

 men in shooting boxes, lie in wait for their prey. Here long ten- 

 tacles usually radiate out from the body in the surrounding water 

 as in Actinobolina radians or in Suctoria, until a victim comes in 

 contact with one or more of the outstretched processes (Fig. 91, 

 p. 163) ; in the same way axopodia of the Heliozoa capture chance 

 organisms which serve as food (Fig. 97). 



The most interesting of these holozoic types are the predatory 

 forms which hunt their prey and capture them, while in full motion. 

 The small but powerful ciliate, Didinium nasutum, belongs in this 

 group. It darts here and there with an erratic movement while 

 rotating at the same time on its long axis. In its sudden darts, 



