76 ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



sisting as it does of an alga and a fungus closely interwoven 

 (fig. 1070). 



Animals and plants may also be associated in an intimate 

 way. It appears, for example, that the process of digestion in 

 many animals is aided by certain bacteria which always live in 

 their internal organs, as, e.g., Sarcina ventriculi in the human 

 stomach (fig. 1073). Bacteria of the sort are provided with a 

 sheltered home and abundant food. Among the Animalcules 

 (Protozoa) a well-known example is afforded by some of the 

 Radiolaria, which always contain so-called " yellow cells", that 

 3 ^ are regarded as a kind of alga (fig. 



1071). These cells are not only shel- 

 tered, but also absorb carbon dioxide, 

 water, and salts from the fluids of the 

 Radiolarian, which in its turn is pro- 

 vided with abundant free oxygen for 

 breathing purposes, and possibly bene- 

 fits in other ways. A somewhat similar 

 association between some Sea - Ane- 

 mones and minute algae has been de- 

 scribed. It is, however, possible that 

 " yellow cells " and " algae " are not 

 plants at all, but specialized parts of the 



Fig. 1070. Cross-section through a Lichen * A 1 i i r* A 



(Coiiema), showing the colourless threads of Kay - Animalcules and oea - Anemones 



the Fungus, and the dark necklace-like fila- i -i 



mentsoftheAlga. X 4 so. UiemSeiVeS. 



PARASITISM. An organism is known 



as a parasite when it feeds upon the substance of another or- 

 ganism, to the serious or fatal detriment of this unwilling "host". 

 An ectoparasite lives on the outside of its host; an endoparasite 

 within it. 



Many plants prey upon other plants in one way or the other. 

 Clover -Dodder {Cuscuta\ for example, is ectoparasitic upon 

 Clover, while various fungi live as endoparasites within higher 

 plants, e.g. Potato- Fungus (Phytophthora infestans] within the 

 tissues of the Potato plant. 



A large number of plants are known which are endoparasitic 

 with regard to animals. In autumn many dead flies will be seen 

 adhering to various objects by a sort of fluffy halo which sur- 

 rounds them. These have been killed by the Fly- Mould (Em- 

 pusa musccz, fig. 1072), the delicate threads of which branch in 



