16 THE PROTOZOA 



into the body of their host, lose their flagella, and nourish them- 

 selves by means of their pigment, which has the nature and proper- 

 ties of plant-chlorophyll ; that is to say, it decomposes carbon dioxide 

 in the sunlight and liberates oxygen. The carbon dioxide is 

 obtained from the respiratory processes of the host, which in its 

 turn utilizes the oxygen produced by the symbionts (p. 197), and 

 thus each organism supplies the needs of the other. When the 

 host enters upon its reproductive processes and breaks up into a 

 vast number of swarm-spores, the symbionts develop flagella and 

 swim off, doubtless to seek for lodging elsewhere. 



It is a matter of convenience to distinguish as epizoic those 

 organisms which live upon, or are attached to, and as entozoic 

 those which live within, the body or substance of the particular 

 form of life with which they are associated. Epizoic forms may 

 be entirely harmless to the creature upon which they occur ; they 

 may simply utilize its body as a coign of vantage where they readily 

 obtain their food, which may consist in some cases of nutritious 

 substances dropped or rejected by the animal that carries them ; 

 or they may obtain the benefits of shelter or transport, especially 

 when the epizoic form in question is itself of sedentary habit. 

 Every naturalist is acquainted with the sea-anemones that live 

 habitually upon hermit-crabs, probably to the advantage of both 

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

 many similar cases among Protozoa. The appendages of Crustacea, 

 especially of the Cladocera and Copepoda, are often thickly beset 

 with sessile Vorticellids and Acinetaria, which obtain a convenient 

 lodging, but provide their own board. Other forms occur similarly 

 on the stems of hydroids, as, for example, Acineta papillifera on 

 Cordylophora lacustris. Amoebae are found creeping on the exterior 

 of Calcareous Sponges, nourishing themselves on diatoms and other 

 organisms. Similar instances could be multiplied indefinitely. 



On the other hand, epizoic forms may be dangerous parasites, 

 nourishing themselves at the expense of the animal they infest, 

 and sometimes inflicting much damage upon it. It can be easily 

 understood that an epizoic form which at first lived harmlessly upon 

 some animal, drawing its supplies of food from the surrounding 

 medium, might acquire the habit ultimately of obtaining its nourish- 

 ment from the living substratum upon which it has planted itself. 

 Examples of epizoic parasites are the flagellate Costia necatrix 

 (p. 272) and the ciliate Ichthyophthirius multifiliis (p. 450), both of 

 which are epizoic parasites of fishes, attaching themselves to the 

 skin and destroying the epidermis ; as a result, the way is left open 

 for fungi and bacteria to penetrate the skin, and so produce ulcera- 

 tion and suppuration, which may be fatal. 



All certain instances of Protozoa acting as external parasites are 



