ECOLOGY, COMMENSALISM AND PARASITISM 359 



pelic and sewage-dwelling fauna. Such forms are partially prepared 

 for survival in the digestive tracts of animals, and these chances 

 are enhanced if their protoplasmic responses to stimuli provide a 

 resistance to the digestive fluids of the animal gut. We know of no 

 case amongst Protozoa where such resistance has been demonstrated 

 as a response to stimuli from the digestive tract, and must go as 

 far afield as the nematode worms for evidence. Here it has been 

 demonstrated that extracts from Ascaris lumbricoides contain anti- 

 ferments which neutralize the digestive ferments of the host (Wein- 

 land, 1902 and 1908). 



Ectoparasitic Protozoa.— An ectoparasitic mode of life in most 

 cases is not sufficiently different from a free-living condition to call 

 for special morphological changes. Attached forms on algae or 

 detritus of different kinds may find an equally good anchorage on 

 shells of molluscs, carapace and appendages of arthropods, gill 

 structures of diverse types of fresh and salt water animals. Such 

 forms have the advantage of moving from place to place with their 

 hosts or of utilizing the food-bearing currents passing over their 

 gills. There is some evidence of adaptation to particular hosts even 

 in these ectoparasitic forms. Thus one can usually find Zooiham- 

 nium affine and Lagenophrys nassa on the legs of Gammarus pulex 

 and Spirochona gemmipara and Dendrocometes 2 )arcl d°> vus on the 

 gill lamellae while other species of the same genera are usually 

 found on Ascllus. In some cases special adaptations for such a 

 mode of life have been developed. Thus the suctorian Trichophrya 

 salparum adheres like a saddle to a gill bar of Salpa (Fig. 100, p. 192) 

 or the vorticellid ciliate Ellobiophrya donacis (Chatton and Lwoff, 

 1929, 1923) in which the usual adhesion disc (as in Scyphidia) is 

 drawn out in two arms which encircle a gill filament of the lamelli- 

 branch Donax vittatus (Fig. 104, p. 202). More frequently an attach- 

 ing organ ("scopula," Faure-Fremiet, 1910) is provided with spines 

 or hooks as in Trichodina species or Cyclochaeta on Hydra. A specific 

 thigmotactic reaction appears to keep Kerona pediculus on the ecto- 

 dermal surface of Hydra fusca. 



Such forms, however, can scarcely be called parasites for they 

 apparently cause no ill-effects on the host. Schroder's term " Plank- 

 tonepibionten," or simply epibionts, appears to be more appropriate. 

 Ectoparasites in a strict sense are rare and appear to be limited to 

 fish hosts where the flagellate Costia necatrix grows to such numbers 

 that vitality, especially of young fish, is greatly impaired. Of the 

 ciliates, Chilodon cyprini furnishes a similar case, while Icthyoph- 

 thirius multifilvm, by boring into the skin of fish, becomes a more 

 deeply-lying parasite and the cause of distributed ulcerations. 



Endoparasitic Protozoa.— In this group adaptations which are 

 often highly complex are mostly physiological and are directed 

 toward the preservation of the individual against the antagonistic 



