FUNCTIONS OF PROTOZOA. 109 



in the contractile filament of the stalk of Vorticella and similar 

 Infusorians ; and not less definite are the movements of cilia 

 and flagella, by means of which most Infusorians travel 

 swiftly through the water. Cilia in movement are bent and 

 straightened alternately, while flagella, which are usually 

 single mobile threads, exhibit lashing movements to and fro. 



Considered generally, the movements are of two kinds : either ( I ) 

 reflex, i.e. responses to external stimulus, as when the Protozoon moves 

 towards a nutritive substance ; or (2) automatic, i.e. such movements as 

 appear to originate from within, without our being able to point to the 

 immediate stimulus, e.g. the rhythmical pulsations of contractile 

 vacuoles. 



Although vital activity or life remains quite untranslatable into 

 lower terms of chemistry and physics, it is useful to compare the 

 movements of Amoelxe with the movements of drops of fine emulsion, 

 as Professor Biitschli has done in great detail. For in this way 

 the strictly vital may be distinguished from what depends on known 

 physical conditions. 



Br. Verworn has speculatively suggested that the substance of the 

 amoeboid cell is drawn out towards oxygen in the medium, that the 

 chemically satisfied particles make way for their unsatisfied neighbour 

 particles, that external stimulus provokes a molecular disruption, and 

 that the exhausted particles have then to retreat to the nucleus, which 

 he regards as a trophic centre. 



Sensitiveness. --The Amoeba is sensitive to external influ- 

 ences. It shrinks from strong light and obnoxious materials ; 

 it moves towards nutritive substances. This sensitiveness 

 is, so far as we know, diffuse a property of the whole of 

 the cell substance ; but the pigment spots of some forms 

 are specialised regions. 



Many Protozoa well illustrate a strange sensitiveness to (the physical 

 and chemical stimuli of) objects or substances with which they are not 

 in contact. Thus the simple amoeboid Vanipyrella will, from a con- 

 siderable distance, creep directly towards the nutritive substance of an 

 Alga, and the plasmodium of a Myxomycete will move towards a 

 decoction of dead leaves, and away from a solution of salt. The same 

 sensitiveness, technically termined chemotaxis, is seen when micro- 

 organisms move towards nutritive media or away from others, when the 

 spermatozoon (of plant or animal) seeks the ovum, or when the phago- 

 cytes (wandering amoeboid cells) of a Metazoon crowd towards an in- 

 truding parasite or some irritant particle. 



Nutrition. --The Ain<xba expends energy as it lives and 

 moves; it regains energy by eating and digesting food 

 particles. Most of the free Protozoa live in this manner 



