PARASITES AS FOOD THIEVES 289 



habiting the alimentary tracts of higher animals. 

 But while there are some quite innocent Protozoa 

 attached to the skin or external covering of other 

 animals, probably no perfectly harmless Protozoon 

 dwells within its host. Even those organisms like 

 some Gregarines and Opalina, that seem to live 

 mainly on waste materials in the food-canals of their 

 hosts, may do injury in two ways. They may absorb 

 small quantities of nourishment intended for their 

 hosts, or they can bring about chemical changes in 

 the effete matter present, which are detrimental to the 

 animal harbouring them. The presence of the ciliate, 

 Blepharocorys, in the food-tracts of horses and cattle 

 causes the generation of gases, and produces intense 

 mechanical irritation if numbers of the organism are 

 present. Again, certain parasites may not only absorb 

 raw food materials intended for their host, but may also 

 utilize some of the products of digestion. A further 

 stage in parasitism is seen in certain Gregarines, 

 as mentioned in a previous chapter. Some merely 

 absorb the fluid digested food from their host's gut, 

 but others absorb the living substance of the host's 

 cells in their neighbourhood. 



Yet other Protozoa are found, forming a large pro- 

 portion of those considered, in which simple entry 

 for shelter has led to occupation for shelter and food, 

 and has ended in complete adaptation to the para- 

 sitic habit. The Amoeba proteus of pond-water is 

 free-living. Entamceba coli absorbs waste materials 

 and digested fluids alike from the human intestine. 

 E. histolytica destroys the cells of the lumen of the 

 alimentary tract, buries itself deep within the sub- 

 19 



