ECOLOGY, COMMENSALISM AND PARASITISM 401 



mouth lies (Fig. 172). Here, also, in addition to the mouth open- 

 ing there are numerous pores opening into the endoplasm through- 

 out the course of the furrow. With this group also we include 

 provisionally Buxtonella, Jameson, a parasite of cattle. 



Among the Trichostomina we have the families Conchoptheriidae 

 which are ectoparasites and endoparasites of molluscs; the Iso- 

 trichidae which are endoparasitic in ruminants; and Cyathodinidae, 

 endoparasites of Covin apersa (see Key, p. 503). 



Amongst the HyfftenostominaT the Ancistridae include ecto- 

 parasites and endoparasiteSn^mussels (Ancistruma, Strand, and 

 Boveria, Stevens) and in holothurians (Boveria, Stevens). 



These forms are all highly modified ciliates which in some cases 

 have acquired characteristics of the Suctoria. Thus Hypocoma 

 possesses a suctorial tentacle which functions as a mouth, and the 

 genera Pelecyophrya , Chatton and Lwoff, and Sphenophrya, Chat- 

 ton and Lwoff, have lost all cilia in the fully-developed condition, 

 their earlier presence indicated only by two zones of basal granules 

 of the infra-ciliature. As in Suctoria reproduction occurs by bud- 

 ding from the dorsal surface, the buds being ciliated, not as are the 

 Suctoria, but like the Ancistridae. 



Amongst the heterotrichs we find the, only ciliated parasites of 

 man represented by species of the genera Xyctotherus and Balan- 

 tidium. Nyctotherus faba, Schaudinn, is a small form which, accord- 

 ing to Reichenow, has been safely identified as an intestinal parasite 

 of man only once (1899) and then in diarrheic stools. Balantidium 

 species are more frequently found in the human intestine; here, 

 particularly in B. coli, the ciliates may run a normal course in the 

 intestine without causing morbid symptoms, but under conditions 

 of the host which are not understood, they may cause an acute 

 enteritis of the same nature as dysentery. Like Endameba, these 

 ciliates may penetrate the gut wall and remain embedded in the 

 deeper tissues. 



Balantidium species are widely distributed amongst the lower 

 vertebrates and mammals and B. coli is a characteristic parasite 

 of the pig, which is the main source of human infection. A second 

 species, B. minutum, was discovered by Schaudinn, together with 

 Xyctotherus, in one case; since then it has been observed only spora- 

 dically (Pinto in Mexico and Mathewossian in Armenia, according 

 to Reichenow-Dofiein, 1929). 



The Oligotrichida are ciliates with greatly reduced filiation, the 

 adoral zone of membranelles and cirri alone representing the motile 

 organs. In the older systems of classification the Order was divided 

 into three families— Halteriidae, Tintinnidae and Ophryoscolecidae, 

 the last including all of the parasitic forms. These parasites are 

 quite different in organization and complexity from other Oligo- 

 trichida, but are of a common type amongst themselves and justify 

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