PROTOZOA AND OTHER ANIMALS 983 



by the addition of protein to the diet (Ferber, 1928; Mowry and Becker, 

 1930), and dedines when protein is deficient, it is evident that, as 

 Mangold (1929) stated, the ciHates have an important protein need. 

 They obtain the protein ordinarily from the plant food. Mangold (1929, 

 1933) thought it unlikely that ingested bacteria or other Protozoa could 

 sufficiently provide for this need. It is normally supplied by the addition 

 of grain. Whether or not the cellulose of the plant food is fully utilized, 

 the starch and protein of the plant cell plasma supply energy and ma- 

 terials for the activity and growth of the Protozoa. 



Many investigators who have concerned themselves with the ciliates 

 of ruminants have sought an answer to the question of their possible 

 value to their hosts. The literature was reviewed by Becker, Schulz, 

 and Emmerson (1930); and the subject has been discussed by Becker 

 (1932) and Mangold (1933). Of the various opinions advanced, 

 Zurn's belief that the ciliates could cause injury has been found entirely 

 untenable. They are present in every ruminant in good condition; and 

 Ferber (1929b) pointed out that the number of ciliates may serve as 

 a guide to the host's well-being. Favorable conditions of nutrition and 

 optimum physiological activity of the host at the same time favor a 

 large ciliate population; and under the best conditions there may be 

 approximately a doubling of the average density, to 2,000,000 per cc. 

 Unfavorable conditions are rapidly reflected in a decline of that popula- 

 tion. 



Some, beginning with the first observers of the ruminant faunule 

 (Gruby and Delafond, 1843), have supposed that the relationship is 

 one of mutualistic symbiosis, the ciliates being in one way or another 

 beneficial to their hosts. It is not disputed that great numbers of the 

 ciliates are digested, but mutualism does not follow from that, unless 

 some special contribution is made to the economy of the host. 



There is still no general agreement as to whether or not the ciliates 

 can break down cellulose, but opinions have been widely published 

 that they aid their hosts in cellulose decomposition. Reichenow (1920) 

 observed morphological changes in ingested cellulose fragments in 

 Troglodytella; and suggested that the significance of this ciliate to its 

 primate hosts, as well as that of Ophryoscolecidae in ruminants, lies in 

 the use of cellulose in constructing their own easily digestible bodies, 

 which serve as animal nutriment for the mammals. In Doflein (Reiche- 



