io8 PROTOZOA. 



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. ] 



Growth and Reproduction. In favourable conditions, 

 when income exceeds expenditure, the Amoeba or other 

 Protozoon grows ; in reverse conditions, or at the limit of 

 growth, it reproduces. The phenomena of reproduction 

 we will consider in greater detail later on. 



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 proto- 

 plasm 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 Euglena, and in many Ciliata, as Stentor^ 

 Sty Ionic hia, one species of Paramcecium, Volvox and the allied forms, 

 are also symbiotic Algce, which have lost the power of independent 

 existence. The evidence for this is, however, insufficient, and this 

 explanation will not apply to cases like that of Vorticella viridis, 

 where the green colouring matter is uniformly distributed through the 

 protoplasm. In many cases there is, besides the chlorophyll, a brown 

 pigment, identical with the diatomin of Diatoms. In many of the 

 Flagellata there are one or more bright pigment spots at the anterior 

 end of the cell ; these may be specially sensitive areas. In some of the 

 simpler Gregarines the medullary protoplasm is coloured with pigment 

 which is apparently a derivative of the haemoglobin of the host. 



Psychical Life. As to the psychical life of the Protozoa, 

 we find that they often behave in a way which suggests con- 

 scious effort and intelligence, but as cut-off fragments also 

 act with apparent reasonableness, and as the nucleus 

 cannot be regarded as a brain, there seems no reason to 

 credit them with more than that diffuse consciousness which 

 is possibly co-extensive with life. Verworn has decided, 

 after much labour, that the Protozoa do not exhibit what 

 even the most sanguine could call intelligence, but this is 



