2l8 L. R. CLEVELAND. 



other individuals containing wood and protozoa. The larvae must 

 become infested \vith protozoa, by eating the feces of older and 

 infested members of the colony, before they can maintain them- 

 selves on a strict wood diet. The host procures the wood for the 

 parasite, and the parasite digests it for itself and for its host. 

 Each is a servant to the other; the protozoa are dependent on 

 the termites for food and lodging, and the termites are dependent 

 on the protozoa for the protozoal cellulose digestion products. 



GENERAL DISCUSSION. 



The number of protozoa present in a sigle host is truly enorm- 

 ous. Here we have an animal whose intestine is completely 

 filled with flagellate protozoa, the majority of which live in a 

 symbiotic relation to their host. Perhaps many if not all- 

 intestinal flagellates human as well as animal are harmless to 

 their host. At least their presence does not mean anything 

 harmful or otherwise. Only careful and painstaking investiga- 

 tion will reveal their relation to their host. 



The examples of symbiosis in nature hitherto described, such 

 as algae and fungi (forming lichens), hydra and Zoochlorellee, 

 sea-anemone and hermit crab, are well known. These relation- 

 ships are much closer than such relationships as ants and aphids, 

 insects and flowers, etc. The symbiosis exhibited by termites 

 and their intestinal protozoa is as real or true as any yet described. 

 How such a relationship was developed is an extremely interest- 

 ing but very difficult question to consider or even theorize * 



1 Since none of the protozoa of termites are known to occur elsewhere it is in- 

 teresting to speculate on their origin. Where did these insects get them? The 

 ability of soil protozoa to digest cellulose has not been studied and little is known 

 regarding the protozoa of plants. If there are protozoa parasitic on plants, or 

 protozoa in the soil, capable of digesting cellulose, it is possible that some of them 

 became inhabitants of termites. It may be that the symbiosis was established in 

 this way. These insects, before harboring protozoa, fed on soil and wood and, per- 

 haps, were able to utilize some of the substances in the wood and many of those in 

 the soil. The intestine of termites, feeding on practically the same food that the 

 protozoa free in nature feed on, is certainly a much more constant and hence 

 better environment for the protozoa than soil, or even plants. But, if the termites 

 once possessed the ability to digest cellulose, why did they lose it? If the acquired 

 parasites had been digesting cellulose longer than the termites and could do it 

 better, i.e., more completely, this may be responsible for the loss of tin- ability to 

 digest cellulose. The insects probably lost their ability to secrete i-n/yiiu-s, such as 

 ccllulase and cellobiase, because it became unnecessary for them to do it. How- 

 ever, it is also just as reasonable to suppose that the termites nevi-r po d tin- 

 ability to digest cellulose, since so few animals do. 



