THE ORGANIZATION OF THE PROTOZOA 



61 



supplying the contractile vacuole or vacuoles. Thus, in Stentor 

 (Fig. 8) the single vacuole is fed by a canal running the length of 

 the Tbody, and in Paramecium (Fig. 185) the two vacuoles are each 

 surrounded by a number of canals forming a star-shaped figure. 



As regards the function of the contractile vacuoles, it should be 

 noted in the first place that their contents are always fluid and 

 watery, and never contain solid particles of any kind. The fluid 

 which a contractile vacuole drains from the body is doubtless 

 replaced by water absorbed from the surrounding medium by 

 diffusion through the superficial layer of the protoplasm, or it 

 may be through the mouth in some cases. The contractile vacuole 

 is generally regarded as the organ of 

 nitrogenous excretion, comparable 

 functionally to the urinary organs of 

 the Metazoa, but it is highly probable 

 that the liquid discharged from it 

 contains also the carbon dioxide pro- 

 duced by the respiratory process. 

 Hence the contractile vacuole may 

 be regarded as both excretory and 

 respiratory in function (see also 

 p. 197, infra). 



4. In the majority of Protozoa 

 there are no organs for which a defi- 

 nite sensory or nervous function can 

 be claimed, although these organ- 

 isms show by their reactions to the 

 environment or to stimuli that they 

 possess sensory and psychical func- 

 tions. In some cases, however, 

 certain organs can be asserted to 

 have a sensory function, exhibited 

 in sensitiveness either to impressions 



of touch or light. Thus, in many Flagellates the flagella appear to 

 be tactile as well as locomotor in function, and in Ciliata tactile 

 cilia occur, especially in the creeping hypotrichous forms. 



Sensitiveness to light is a marked feature of many Protozoa, 

 even of quite undifferentiated forms, such as amoebae. Rhumbler 

 (34) has shown that many amoebae cease feeding in a strong light, 

 and even disgorge food that they have taken in when suddenly 

 subjected to the intense illumination necessary for microscopic 

 study. This characteristic is, however, most marked in the holo- 

 phytic species, to which light is a necessity for their plant-like 

 metabolism. In the holophytic Flagellates a red pigment-spot, or 

 stigma, is found constantly, situated close to the anterior end of the 



FIG. 31. Pouchetia cornuta, one of 

 the Dinoflagellata, to show the 

 large stigma (st.), in front of 

 which is a lens (I.). After Schiitt 

 (386). 



