104 PROTOZOA THE SIMPLEST ANIMALS. 



ensues. In all Protozoa 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 Zoochlorella. According to some, the 

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

 flagellate forms, as Euglena, and in many Ciliata, as Stentor, Stylo- 

 nichia, one species of Paramecium, Volvox and the allied forms, are 

 also symbiotic Algae, which have lost the power of independent exist- 

 ence. The evidence for this is, however, insufficient, and this explana- 

 tion will not apply in 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. — Protozoa often behave in a way which 

 suggest conscious control 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 conscious- 

 ness which is possibly co-extensive with life. Verworn has 

 decided, after much labour, that the Protozoa do not exhibit 

 what even the most generous could call intelligence ; but this 



