THE president's ADDRESS. 



state of our knowledge. I propose to lay before you a few 

 facts of parasitism, restricting myself entirely to the field of 

 the Protozoa, and to consider the phenomena solely from the 

 point of view of natural history; a method of treatment which,, 

 though it cannot claim to solve all difficulties, may serve 

 nevertheless to furnish certain guiding principles, and to throw 

 some light, if only a side-light, on the questions at issue. 



Amongst Protozoa, as is well known, many different modes- 

 of life occur. We are concerned here with those species which 

 live constantly in association with some other organism of a 

 distinct kind. Such a species may live either upon, or in, the 

 organism with which it is associated ; in the first case it is 

 then termed epizoic, in the second entozoic. In all such cases 

 of association, the species which are truly parasitic must be 

 clearly distinguished from those which are not. Taking first 

 the case of epizoic forms, it is found that in many cases they 

 merely utilise the body of their host as a coign of vantage where 

 they readily obtain their food, or as a convenient means of trans- 

 port, especially when the epizoic form in question is of a sessile habit 

 of life. Every naturalist is acquainted with the sea-anemones 

 that live upon hermit-crabs, to the advantage probably of 

 both animals, at all events to the detriment of neither. There 

 are many similar cases amongst the Protozoa. As every micro- 

 scopist knows, the appendages of many Crustacea, especially 

 of Cladocera, such as Daphnia, and Copepoda, such as Cyclops y 

 are often thickly beset with sessile Vorticellids or Acinetans^ 

 which obtain a convenient lodging but provide their own board. 

 Other forms occur on the stems of Hydroids, as for example 

 Acineta papillifera on Cordylophora lacustris (Hickling Broad, 

 Norfolk). The common Hydra often bears a beautiful Infusorian 

 which from its louse-like appearance has the name Trichodina 

 2)edicfdus, but which caters for itself and not at the expense of 

 the Hydra. Amoebae are to be found creeping on the exterior 

 of calcareous sponges, nourishing themselves on diatoms and 

 other organisms. Similar instances could be multiplied in- 

 definitely of epizoic species which are merely commensals, and 

 not parasites in any sense of the word. 



