no PHYLUM PROTOZOA- -THE SIMPLEST ANIMALS. 



upon solid food particles ; a few, such as Volvox^ in virtue 

 of their chlorophyll, are holophytic, i.e. they feed like plants ; 

 the parasitic forms usually absorb soluble and diffusible 

 substances from their hosts. 



Eespiration. Like all living creatures, the Amoeba re- 

 spires, that is, its complex substance is continually under- 

 going a process of oxidation, carbon dioxide being produced 

 as a waste product. Without oxygen none of the activities 

 can be efficiently performed, and if it is long withheld death 

 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 Algce, so-called Zoochlorellce. According to some, the 

 "chlorophyll corpuscles" seen in the primitive Archer/net, in some 

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

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

 also symbiotic Algre, 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 diatoinin 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. 



