I THE PROTOZOA 21 



which has been absorbed. Plants are only able to perform 

 this process of obtaining and assimilating carbon from the 

 air, by virtue of the green pigment chlm'(/phyll, which is 

 characteristically, though not invariably, present in them 

 and absent in animals. Such nutrition, dependent on the 

 presence of chlorophyll, is termed holophytic. 



Further, owing to the nature of the plant food, and to its 

 presence throughout the plant environment, it can be absorbed 

 by the process of diffusion, through the whole surface 

 of the unicellular organism, and there is no need for any 

 special organs for obtaining food, or for any mouth or gullet 

 to convey the food to the centre of the protoplasmic body ; 

 hence the surface of the unicellular plant is usually completely 

 surrounded by a permeable cell wall of the substance cellulose, 

 which is secreted by the protoplasm. Such a cell wall is found 

 in very few animal cells, the outer protective coat, when 

 present in animal organisms, being formed generally of 

 calcium carbonate (chalk), or of silica, or of a chitinous 

 substance. 



Power of Locomotion is possessed by unicellular plants and 

 animals alike, so that — although the presence or absence of 

 this is a striking difference between the higher plants and 

 animals — it fails us entirely as a criterion here. Again, a 

 contractile vacuole is more usual in unicellular animals than 

 in plants. 



Pkactical Work on Protozoa 



1. Mount a drop of water containing AmoeJxu'^ on a slide and 

 cover with a cover-slip. Examine under the low power of a 

 microscope (Zeiss A gives a convenient magnification). From amongst 

 the debris in the water, an Amoeba may in time be seen 

 emerging. Watch its movements, and draw it at intervals, showing 

 a few of the various shapes it assumes. Examine under a high 

 power, and find the nucleus, contractile vacuole, and food vacuoles. 

 If possible, watch an Amoeba catching its food. Look for stages 

 in division. Touch it and see it contract. 



2. Examine a prepared slide of Foraminifera in which there 

 are unbroken shells ^ ; also mount a little of the dust rubbed from 



^ These can be obtained in Is. bottles from T. Bolton, 25 Balsall Heath 

 Road, Birmingham. 



2 May be obtained from W. Watson and Sons, 313 High Holborn, London, 

 or from C. Baker, New Oxford Street, London, or from any other dealer in 

 microscope accessories. 



