FUNCTIONS OF PROTOZOA. 103 



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

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

 vacuoles. 



While all vital activity or life must remain inexplicable in lower 

 terms until we know the chemical nature of protoplasm, it is useful to 

 compare the movements of Amcebte with the movements of drops of 

 fine emulsion, as Professor Blitschli has done in great detail. For in 

 this way the strictly vital may be distinguished from what depends on 

 known physical conditions. 



Dr. 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 legards 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 amceboid Vampyrella 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 termed 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 amceboid cells) of a Metazoon crowd towards an in- 

 truding parasite or some irritant particle. 



Nutrition. — The Amxba 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 

 upon solid food particles ; a few, such as Volvox, in virtue 

 of their chlorophyll, are holophytic, i.e. they feed like plants ; 

 the parasitic forms usually absorb soluble and diffusible 

 substances from their hosts. . 



Respiration. — Like all living creatures, the Amxba re- 

 spires, that is, its complex substance is continually under- 

 going a process of oxidation, carbon dioxide being produced 

 as a waste product. Without oxygen none of the activities 

 can be efficiently performed, and if it is long withheld death 



