894 PROTOZOA AND OTHER ANIMALS 



provided by accidental and facultative parasitism. Among Protozoa this 

 is best exemplified by certain holotrichous ciliates. As the subject de- 

 velops, instances in which certain genera contain both free-living and 

 symbiotic species come under consideration, together with examples of 

 closely related genera in the two types of habitat. The situation may be 

 illustrated by certain euglenid and some polymastigote flagellates, and 

 again by certain holotrichous ciliates. In certain groups of Protozoa, in- 

 structive series in degree of adaptation and types of relationship appear. 

 One of the most fruitful of these series, in which the range is from 

 incidental commensalism to strict parasitism, is found in the Thig- 

 motricha. Host relationships of a variety of types, which bring out also 

 the adaptation of life cycles to the conditions of symbiotic existence, 

 are found among certain holotrichs of the marine Crustacea, Conidio- 

 phrys pilisuctor, and the Apostomea. There is more or less morpho- 

 logical alteration and adaptation in the symbiotic holotrichs considered 

 above, particularly in the Thigmotricha; and this is also well brought 

 out in the development of attachment structures in Ptychostomidae, 

 Astomata, peritrichs, and certain flagellates of termites. 



Animals of certain specific groups are characterized by protozoan 

 faunules of particular types. That does not express only the common- 

 place fact that examination of particular hosts will reveal particular 

 symbionts, but makes the point that related hosts often have faunules 

 of similar composition. In this matter of distribution of symbionts 

 among species of hosts is incorporated the problem of specificity in 

 symbiosis, the quality of a symbiont of being restricted to certain hosts. 

 This quality is known as host-specificity. The incorrectness of the term 

 "host-parasite specificity" has been commented on by Hertig, Talia- 

 ferro, and Schwartz (1937). Faunules of two groups of animals are 

 discussed here in their composition, host-specificity, and distributional 

 characteristics: the ciliates of sea urchins, in which there is in part rela- 

 tionship to free-living species; and the Protozoa of termites and Crypto- 

 cercus, in which the symbiotic relationship has reached a maximum de- 

 velopment. 



Finally, physiological host relationships are discussed in detail in 

 two groups of Protozoa: Ophryoscolecidae in ruminants, and flagellates 

 in termites and Cryptocercus. The former relationship, though long sus- 

 pected of being mutualism, is probably simple commensalism; there is 



