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, eg., Sarvcina ventricui 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 
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 alge has been de- 
scribed. It is, however, possible that 
“yellow cells” and “alge” are not 
; plants at all, but specialized parts of the 
Fig. 1070. —Cross-section through a Lichen : 
(Collema), showing the colourless threads of Ray-Animalcules and Sea-Anemones 
the Fungus, and the dark necklace-like fila- 
ments of the Alga. 450. themselves. 
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 exdoparasite 
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, eg. 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 (Am- 
pusa musc@, fig. 1072), the delicate threads of which branch in 
