I REACTIONS TO STIMULI 21 



(iii.) Eheotaxy. — This is the tendency to move against 

 the stream in flowing water. It is shown hy most Protists, and 

 can be conveniently studied in the large amoeboid plasmodia 

 of the Myxomycetes, which crawl against the stream along wet 

 strips of filter paper, down which water is caused to flow. Most 

 animals, even of the highest groups, tend to react in the same way ; 

 the energetic swimming of Fishes up-stream being in marked 

 contrast with their sluggishness the other way; and every 

 student of pond-life knows how small Crustacea and Kotifers, no 

 less than Ciliates, swim away from the inrush of liquid into the 

 dipping- tube, and so evade capture. (See Vol. II. p. 216.) 



(iv.) The movements of many Protozoa are affected greatly by 

 Light. These movements have been distinguished into " photo- 

 pathic," i.e. to or from the position of greatest luminosity ; and 

 " phototactic," along the direct path of the rays.^ Those Protozoa 

 that contain a portion of their cytoplasm, known as a " plastid " or 

 " chromatophore " (see pp. 36, 39), coloured by a green or yellow 

 pigment are usually "phototactic." They mostly have at the 

 anterior end a red pigment spot, which serves as an organ of sight, 

 and is known as an " eye-spot." In diffused hght of low intensity 

 they do not exhibit this reaction, but in bright sunlight they 

 rise to the surface and form there a green or yellow scum. 



Most of the colourless Protista are negatively phototactic or 

 photopathic ; but those which are parasitic on the coloured ones 

 are positively phototactic, like their hosts. 



Here, as in the case of other stimuli,^ the absolute intensity 

 of the light is of importance ; for as it increases from a low 

 degree, different organisms in turn cease to be stimulated, and 



1 It is not always easy to distinguish these two classes of phenomena. 



2 Jennings, in his studies on Emctions to Stimuli in Unicellular Organisms 

 (1899-1900), has shown that whatever be the nature of the repellent stimulus, 

 chemical or mechanical or thermal, the reaction of Paramecium and many other 

 Protista is always the same. It swims backward a short distance, turns towards 

 the aboral surface, and then having thus reversed swims on again in the new 

 direction, front foremost as before. Apparent "positive taxies" are often really 

 negative ones ; for if the Paramecium be placed in water containing COj it shows 

 the reaction not on entering the part charged with this acid, but on passing away 

 from it into purer water, so that it continually tends to turn back into the acid 

 part, while within it or in the water at a distance not yet charged it swims about 

 irregularly. It appears due to this that the individuals become aggregated 

 together, as they excrete this gas into the water. If a repellent substance diffuse 

 towards the hinder end of a Parameciiim, the response, instead of carrying it away, 

 brings it into the region of greater concentration, and may thus kill it. 



