892 PROTOZOA AND OTHER ANIMALS 



usage. Hertwig (1883) had a similar understanding of the meaning 

 of the word; parasitism and mutuahsm, he stated, are types of symbiosis. 

 As the Committee pointed out, he changed his usage later. 



In recent literature other opinions that the words should be used in 

 this original sense have appeared. W. Schwartz (1935) took that atti- 

 tude, although he would restrict symbiosis to the relationship in which 

 there is physiological dependence of one partner on another. Cleve- 

 land (1926) remarked that it would be much better to use it in the gen- 

 eral sense, if the change could be made. 



Van Beneden (1876) referred to certain associated animals as mutual- 

 ists, before the term symbiosis had been coined. The word conveys the 

 idea of reciprocal benefit, although the examples he described indicate 

 a vague concept on his part of the relationships concerned, and none of 

 them would now be regarded as mutualistic. (He discussed among 

 mutualists parasitic copepods, opalinids, endozoic rotifers, and even 

 Vaginicola on Gammarus) . He recognized the three types of associa- 

 tion: commensalism, mutuality, and parasitism. If symbiosis is used 

 in the broad sense, reciprocal relationships should be termed mutualism 

 or mutualistic symbiosis. 



Protozoa that live in natural cavities of the body, such as the mantle 

 cavities of molluscs and the lumen of the alimentary canal, but do not 

 nourish themselves at the expense of the host, have been termed in- 

 quihnes (Caullery, 1922; Grasse, 1935). All inquilines are commensals, 

 but not all commensals are inquilines. There are also ectozoic com- 

 mensals, or ectocommensals; endocommensalism is equivalent to in- 

 quilinism. In a sense, inquilines, like ectozoic symbionts, have not in- 

 vaded the body itself. They occupy cavities open to the outside. 



Endozoic Protozoa which invade the interior of the body proper, 

 living intracellularly, among tissue cells, or in blood or coelomic cavi- 

 ties, are all parasitic. The conditions of their nutrition necessarily involve 

 strict dependence on the host. Ectozoa and inquilines, which usually are 

 commensals, may become parasites when in their nutritive processes 

 they develop one or another means of using the substance of the host, 

 generally by attack upon, or extraction of substance directly from, the 

 epithelial cells. They are also called parasites when they consume enough 

 material that would otherwise be used by the host to make a difference 

 to it; or when in some way not connected with nutrition they injure the 



