114 PROTOZOOLOGY 



ferent circumstances. Many investigators have, up to the present 

 time, studied the reactions of various Protozoa to external stimula- 

 tions, full discussion of which is beyond the scope of the present 

 work. Here one or two examples in connection with the reactions 

 to each of the various stimuli will only be mentioned. Of various 

 responses expressed by a protozoan against a stimulus such as 

 changes in body form, movement, structure, behavior, etc., the 

 movement is the most clearly recognizable one and, therefore, free- 

 swimming forms, particularly ciliates, have been the favorite ob- 

 jects of study. We consider the reaction to a stimulus in pratozoans 

 as the movement response, and this appears in one of the two direc- 

 tions: namely, toward, or away from, the source of the stimulus. 

 Here we speak of positive or negative reaction. In forms such as 

 Amoeba, the external stimulation is first received by the body sur- 

 face and then by the whole protoplasmic body. In flagellated or 

 ciliated Protozoa, the flagella or cilia act in part sensory; in fact in a 

 number of cihates are found non-vibratile cilia which appear to be 

 sensory in function. In a comparatively small number of forms, there 

 are sensory organellae such as stigma, ocellus, statocysts, concretion 

 vacuoles, etc. 



In general, the reaction of a protozoan to any external stimulus 

 depends upon its intensity so that a certain chemical substance may 

 bring about entirely opposite reactions on the part of the protozoans 

 in different concentrations and, even under identical conditions, 

 different individuals of a given species may react diffferently. 



Reaction to mechanical stimuli. One of the most common stimuli 

 a protozoan would encounter in the natural habitat is that which 

 comes from contact with a solid object. When an amoeba which 

 Jennings observed, came in contact with the end of a dead algal 

 filament at the middle of its anterior surface (Fig. 48, a), the amoe- 

 boid movements proceeded on both sides of the filament (5), but 

 soon motion ceased on one side, while it continued on the other, and 

 the organism avoided the obstacle by reversing a part of the current 

 and flowing in another direction (c). When an amoeba is stimulated 

 mechanically by the tip of a glass rod (d), it turns away from the 

 side touched, by changing endoplasmic streaming and forming new 

 pseudopodia (e). Positive reactions are also often noted, when a 

 suspended amoeba (/) comes in contact with a solid surface with the 

 tip of a pseudopodium, the latter adheres to it by spreading out (g). 

 Streaming of the cytoplasm follows and it becomes a creeping form 

 (h). Positive reactions toward solid bodies account of course for the 

 ingestion of food particles. 



