Phylum Protozoa 351 



two different forms live together in a manner which is beneficial to 

 both. Among the protozoans there are many such examples, includ- 

 ing several species of flagellates which live as symbionts in the in- 

 testine of the termites. These forms apparently are able to digest the 

 cellulose which the termites ingest — the termites themselves have no 

 enzymes for cellulose digestion. In return, the termites furnish shelter 

 and food for the protozoan guest. Commensal relationships exist when 

 two organisms live together but neither one benefits nor is injured 

 by the association. Certain sessile protozoans that are attached to the 

 backs of turtles illustrate this. Finally there is the condition where one 

 form gets all the benefits of the relationship while the other receives 

 none or is damaged. This is known as parasitism. The protozoan 

 that causes malaria in man is an example of this latter mode of living. 



Many species of the protozoans have special tests or shells which 

 enclose the protoplast. Often these are of very complicated form and 

 have various sculpturings on them. They are formed of many differ- 

 ent materials such as an organic matrix with imbedded foreign mate- 

 rials, silica, calcium carbonate, or a complex organic material known 

 as tectin. 



Many parasitic forms are able to withstand adverse circumstances 

 and to spread from host to host by the formation of resistant spores. 

 There are some parasitic and free-living forms which are able to pro- 

 duce cysts. These are resistant cells in which the protoplasm is re- 

 organized and a heavy protective wall is formed. Some of these can 

 withstand temperatures as high as 106° C. and as low as that obtained 

 by liquid air. These cysts which can withstand long periods of desicca- 

 tion probably account for the world-wide distribution of many species 

 of protozoans. The extremely tiny cysts can be wafted great distances 

 by air currents. 



Reproduction among the protozoans may be either sexual or asex- 

 ual. Asexual reproduction is the commonest and may be either binary 

 or multiple fission. In binary fission the animal divides into two equal 

 parts ; in multiple fission the single protoplast divides to produce many 

 like individuals. This latter is particularly common among parasitic 

 forms. Budding occurs among some larger forms. In this, the bud is 

 normally unlike the adult which produced it. It may or may not be of 

 the same size. 



Sexual reproduction may be one of two types : copulation or 

 syngamy and conjugation. In copulation or syngamy, there is fusion 



