FUNCTIONS OF PROTOZOA. 97 



Ordinary Functions. — The Amaba draws in one part of its 

 cell-substance and protrudes another, — it is contractile; it 

 moves apparently " of itself," and shrinks from strong light 

 and obnoxious materials, — it is " automatic and irritable " ; it 

 makes up for the energy thus expended by eating and 

 digesting food-particles, — it is '•'-receptive and assimilative"; 

 within its cell-substance by-products are made, which are 

 retained for further use {secretions), or pass out as waste 

 {excretions). It is an essential condition of its life, that 

 oxygen be supplied (for life involves an oxidation of the 

 living matter), and that the waste carbon dioxide be got rid 

 of; in other words, the Amoeba respires. Moreover, when its 

 income exceeds its expenditure, it grows ; in reverse con- 

 ditions, or at the limit of growth, it reproduces. We have 

 already cited the account of these functions given in the first 

 chapter of the fourth edition of Foster's Physiology. There it 

 is also clearly shown how the Amoeba, discharging all its func- 

 tions within the compass of a cell, differs from a higher animal 

 in which distinct sets of cells have been specialised for 

 various activities. The Protozoon is structurally or mor- 

 phologically simple, but in one sense it is, just because of 

 this simplicity, physiologically complex. All the functions 

 are crowded into the activity of a unit mass, whereas a 

 cell in a higher animal has usually one function dominant 

 over the others. It is therefore easier to study the phy- 

 siology of higher animals, while it is difficult to find out 

 much in regard to the contraction, digestion, and other 

 functions in the Protozoa. 



The blunt or thread-like outflowings of Rhizopods are 

 usually associated with streaming movements of the cell-sub- 

 stance, the granules passing inwards and outwards. A some- 

 what similar streaming may be seen within the cell, both in 

 Protozoa and in some plants. A defined contraction, like that 

 of a muscle-cell, is well illustrated in the stalk of Vorticella 

 and some similar Infusorians. The numerous cilia which 

 lash most Infusorians through the water are " bent and 

 straightened alternately," while the flagellum, which is usually 

 a single motile thread, " exhibits lashing movements to and 

 fro, and is thrown into serpentine waves during these move- 

 ments." 



That intra-cellular digestion occurs, that respiration is 



G 



