FUNCTIONS OF PROTOZOA. 113 



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 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 amoeboid cells) of a Metazoon crowd towards an in- 

 truding parasite or some irritant particle. 



Nutrition. The Amoeba 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. 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. 



Excretion. Of the details of this process little is certainly 

 known, but the contractile vacuoles are, without doubt, 

 primitive excretory appliances. In the more specialised 

 forms they appear to drain the cell substance by means of 

 fine radiating canals, and then to burst to the exterior. 

 Uric acid and urates are said to be demonstrable as waste 

 products. 



Colour. Pigments are not infrequently present in the Protozoa, 

 We have already noticed the presence of chlorophyll in some forms ; 

 with Radiolarians the so-called "yellow cells" are found almost 

 constantly associated. Each of these cells consists of protoplasm, 

 surrounded by a cell wall, and containing a nucleus. The protoplasm 

 is impregnated with chlorophyll, the green colour of which is obscured 

 by a yellow pigment. Starch is also present. The cells multiply by 

 fission, and continue to live after isolation from the protoplasm of the 

 Radiolarian. All these facts point to the conclusion that the cells 

 are symbiotic Algae, so-called Zoochlorellce. According to some, the 

 "chlorophyll corpuscles" seen in the primitive Archerina, in some 

 flagellate forms, as 'Eughna, and in many Ciliata, as Stentor, Stylo- 

 nicliia. one species of Paramcecium^ Volvox and the allied forms, are 



