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 amceboid cells) of a Metazoon crowd towards an in- 
truding parasite or some irritant particle. 
Nutrition.—The Amada 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, ze. 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. BS get 
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 conclusior that the cells 
are symbiotic Algze, so-called Zoochlorelle. According to some, the 
“chlorophyll corpuscles” seen in the primitive Avcherina, in some 
flagellate forms, as Huglena, and in many Ciliata, as Stentor, Stylo- 
nichia, one species of Paramacium, Volvox and the. allied forms, are 
