862 LIFE : OUTLINES OF GENERAL BIOLOGY 



some artificiality in the environment. As regards the abundance of 

 larger parasites in many animals, there is no doubt that a live-and- 

 let-live compromise has often been reached between host and guest. 

 The grouse always contains hundreds, if not thousands, of minute, 

 almost transparent, threadworms in its intestine, but these are not 

 known to be of serious moment unless the vigour of the bird is 

 diminished by prolonged bad weather, insufficient food, overcrowding 

 and its consequences. Then the multiplication of the previously 

 unimportant threadworms, along with other parasites, may become 

 prodigious and fatal. 



How do Protozoa stand as regards the attacks of virulent microbes ? 

 Experiments have shown that some Protozoa are well able to 

 ingest and digest microbes, and some, such as certain species of 



^^ ^ o 



/ w 



Fig. 146. 



A Colony of Flagellate Infusorians, Proterospongia hoeckelii. After Saville 

 Kent. There are about forty flagellate individuals (FL), each with a 

 nucleus, a wineglass-shaped collar, and a flagellum or lash. In the internal 

 gelatinous matrix there are amoeboid individuals (AM), some of which 

 may be seen in process of division. This may be one of the descendants 

 of transitional forms between Protozoa and Metazoa. 



soil Amoebse, feed largely on Bacteria. This points forward to the 

 microbe-destroying fimction of the wandering amoeboid phagocytes 

 which are present in most multicellular animals. To some extent, 

 then. Protozoa can resist microbic death. 



Natural Death is due to the slow mounting-up of unrepaired 

 results of vital wear and tear. Living implies a transformation of 

 energy within a material system, and some structural disintegration 

 is inevitable. This, as everyone knows, is counteracted by processes 

 of recuperation, as afforded by food, rest, sleep, and change. All the 

 recuperative influences may be summed up in the concept of rejuven- 

 escence — keeping the body young, or making it young again after 

 some exhaustion. From youth onwards these operate against the 

 processes of senescence or ageing, the inevitable consequence of any 

 failure to make good the wear and tear. The more complicated the 

 structure of the body — ^^\'hich some are too fond of calling its 



