1883-84-] Edinburgh Natuyalists' Field Club. 175 



IJonellia viriJis, the Anthea green of Antliea cerens, the Crustacea 

 green of Paleeuion viridis, and the Pentacrinin of Pentacrinus — this 

 last named pigment changing from a green to a purple liue on acidi- 

 fication. Yet green pigment of functional value equivalent to 

 clilorophyll is apparently sometimes developed by true animal pro- 

 toplasm ; while, on the other hand, greenness in animals may be due 

 to the existence of green or greenish- yellow j^lant organisms existing 

 inside the animal tissues, in which case we have the curious phe- 

 nomenon of an animal and a plant agreeing to live together, and, as 

 we shall see below, " reciprocally accommodating " one another from 

 a physiological standpoint. These organisms are said to be Sym- 

 biosists, Commensalists, or Mutualists. 



It is at this stage to be remarked that of this association of one 

 organism with another we find many illustrations, differing at once 

 in kind and in degree. The simplest type is perhaps represented by 

 cases where the guest is external and the host is entirely passive. 

 Thus we find Diatoms epiphytic on Algte; Lichens, Alga3, or Mosses, 

 on trees ; Algse epizoic on Snails, or even on the more active Cyclops 

 and Daphnia ; Avhile the list of Seaweeds found growing on other 

 Algte is a very large and comprehensive one, although the precise kind 

 of this association — whether wholly or partially epiphytic or wholly 

 or partially parasitic — is not in all cases clearly determined. In 

 addition to the fact of association of plant with plant, we find similar 

 associations of animal with animal. Thus the Commissioners on the 

 Fisheries of New South Wales, in a report published in December 

 1883, remark that "the very young fry of Trachurus trachurus have 

 a most extraordinary and ingenious way of providing for their safety 

 and nutrition at the same time. They take up their quarters inside 

 the umbrella of the large Meduste, where they are safe from their ene- 

 mies, and are, without any exertion on their part, supplied with the 

 minute organisms which constitute their food, by the constant current 

 kept up by the action of the curtain-like cilia of the animal." In 

 Fol's 'Recueil Zoologique Suisse,' vol. i. (1883), pp. 65-74, a similar 

 association of Caranx melampygus and Crambessa palmipes is re- 

 corded from the Mauritius. Annelid tubes, too, have been found 

 surrounded by the corallum of Poritidse and other coralline zoophytes. 

 On the other hand, there are associations of organisms in which both 

 host and guest are active and mutually beneficial to one another, as 

 in the case of Anthea, Lichens, &c., to be referred to below. 



The distinction between Commensalists and true parasites, such as 

 the Dodder, Peronospora and other Fungi, Ttenise, Trachinse, 

 Oxyuridse, &c. — all of which exercise a deleterious function upon 

 their host — was first drawn by Van Beneden, who explained the 

 phenomenon of Commensalism by a " sympathy " existing between 

 host and guest. But, more recently, Mr Geddes (' Proc. Roy. Soc.,' 

 Lend., 1879), by submitting a number of green Convoluta Schultzii 



