288 SOME MINUTE ANIMAL PARASITES 



of other animals, etc. They are thus dependent 

 indirectly on other living things for their support. 

 Their mode of nourishment compares well with that 

 of a fungus, such as a mushroom. 



The greatest variation is found among the parasitic 

 Protozoa. While the fungus-like method of nutrition 

 does not, as a rule, depend on any one organism, 

 those Protozoa that have become parasitic are often 

 dependent on some special host or hosts, failing 

 which they perish. Every Protozoon attached to a 

 higher animal is not parasitic. Many fasten them- 

 selves to the outsides of certain Mollusca and worms, 

 and such Crustacea as water-fleas, either by accident 

 or because the position is one of advantage in obtain- 

 ing food or shelter. When sedentary molluscs are 

 used as hosts, shelter is the main end attained, while 

 the active movements of the Crustacea through the 

 water afford their protozoal guests numerous oppor- 

 tunities of obtaining suitable food and possibly extra 

 quantities of oxygen. In neither case does any harm 

 accrue to the host, and it is even possible that there 

 may be advantages. 



On the other hand, the Ichthyophthirius attaches 

 itself to the skin of a fish, and by its action produces 

 pustules and skin decay. The Protozoon alone is 

 sufficient to cause death, but the open wounds that 

 are caused by its attacks allow of the entry of 

 bacteria and of fungi, such as the deadly Saprolegnia, 

 and so hasten the end of the host. Thus Protozoa 

 living on the outside of animals may be either harm- 

 less or extremely dangerous. 



Similar effects follow the action of Protozoa in- 



