ECOLOGY 29 



on various species of Hydra, and many ciliates placed in Thigmo- 

 tricha (p. 774) are inseparably associated with certain species of 

 mussels. 



Endocommensalism is often difficult to distinguish from endo- 

 parasitism, since the effect of the presence of a commensal upon the 

 host cannot be easily understood. On the whole, the protozoans 

 which live in the lumen of the alimentary canal may be looked upon 

 as endocommensals. These protozoans undoubtedly use part of the 

 food material which could be used by the host, but they do not in- 

 vade the host tissue. As examples of endocommensals may be men- 

 tioned: Endamoeba blattae, Lophomonas blattarum, L. striata, 

 Nyctotherus ovalis, etc., of the cockroach; Entamoeba coli, Iodamoeba 

 biitschlii, Endolimax nana, Dientamoeba fragilis, Chilomastix mes- 

 nili, etc., of the human intestine; numerous species of Protociliata of 

 Anura, etc. Because of the difficulties mentioned above, the term 

 parasitic Protozoa, in its broad sense, includes the commenals also. 



Symbiosis on the other hand is an association of two species of 

 organisms, which is of mutual benefit. The cryptomonads belonging 

 to Chrysidella ("Zooxanthellae") containing yellow or brown chrom- 

 atophores, which live in Foraminifera and Radiolaria, and certain 

 algae belonging to Chlorella ("Zoochlorellae") containing green 

 chromatophores, which occur in some freshwater protozoans, such as 

 Paramecium bursaria, Stentor amethystinus, etc., are looked upon 

 as holding symbiotic relationship with the respective protozoan host. 

 Several species of the highly interesting Hypermastigina, which are 

 present commonly and abundantly in various species of termites and 

 the woodroach Cryptocercus, have been demonstrated by Cleveland 

 to digest the cellulose material which makes up the bulk of wood- 

 chips the host insects take in and to transform it into glycogenous 

 substances that are used partly by the host insects. If deprived of 

 these flagellates by being subjected to oxygen under pressure or to 

 a high temperature, the termites die, even though the intestine is 

 filled with wood-chips. If removed from the gut of the termite, the 

 flagellates perish (Cleveland, 1924, 1925). Recently, Cleveland 

 (1949-1950c) found that the molting hormone produced by Crypto- 

 cercus induces sexual reproduction in several flagellates inhabiting 

 its hind-gut (p. 185). Thus the association here may be said to be an 

 absolute symbiosis. 



Parasitism is an association in which one organism (the parasite) 

 lives at the expense of the other (the host) . Here also ectoparasitism 

 and endoparasitism occur, although the former is not commonly 

 found. Hydramoeba hydroxena (p. 464) feeds on the body cells of 



