ORDINARY FUNCTIONS. 107 



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

 physical conditions. 



Dr. Verworn has speculatively suggested that the substance of the 

 amceboid 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 Amceba is sensitive to external influ- 

 ences. It shrinks from strong Hght and obnoxious materials, 

 it moves towards eatable 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 Ckenwtaxis, 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 (viiandering amceboid cells) of a Metazoon crowd towards an intrud- 

 ing parasite or some irritant particle. 



Nutrition. — The energy which the Amaba expends in 

 movement, it makes up for by eating and digesting food 

 particles. Most of the free Protozoa live in this manner 

 upon solid food particles, whether plant or animal ; a few 

 such as Volvox, in virtue of their chlorophyll, live entirely 

 as do plants ; the parasitic forms usually absorb soluble and 

 diffusible substances from their hosts. 



Respiration. — Like all living creatures the Amxba respires, 

 that is, its complex substance is continually undergoing 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 

 ensues. In all Protozoa oxygen is simply taken up by the 

 general protoplasm from the surrounding medium, into 

 which the waste carbonic acid is again passed. The 

 bubbles which enter with the food particles assist in 

 respiration. In parasitic forms the method of respiration 

 must be the same as that of the tissue cells of the host. 



