THE RADIOLARIA 129 



never leave it. They become assimilating granules, apparently 

 incapable of independent life, and are transmitted from parent to 

 offspring. In the former conclusion we have a remarkable parallel 

 to the history of the green cells (zoochlorellae) in Convoluta roscoffensis 

 (Keeble and Gamble [41]). 



The nature of these interesting zooxanthellae is not satisfactorily 

 settled, but the scanty evidence points to affinities quite distinct 

 from those of the other Radiolaria. In the absence of a knowledge 

 of the life-history, Brandt's view is as likely as any other, but it is 

 by no means certain that all the Acantharian zooxanthellae are 

 of similar parentage. This view is that the zooxanthellae of 

 Acanthoniidae and Dorataspidae are isolated spindles of Labyrinthula 

 vitellina or of some allied species, and Brandt (10&, p. 239) points 

 out the agreement between the two structures in their shape, size, 

 colouring, and nuclei. 



The association between Kadiolaria and the zooxanthellae is 

 usually regarded as a symbiosis, i.e. one of mutual advantage. It 

 is, however, clear from the foregoing description that no single 

 formula will cover the important facts (1) that we have degrees of 

 intimacy that have grown up between the two organisms; and (2) 

 that the last term in the series of association is one to which 

 symbiosis in any but the widest sense of that term is inapplicable. 



The origin of the association is probably to be traced to the 

 hunger for nitrogen on the part of the zooxanthellae ; to the 

 minimal quantity of inorganic nitrogenous food-stuffs in the warmer 

 seas (Johnstone [45]) ; and to the convergent adaptation of 

 Radiolaria and zooxanthellae to life at or near the surface of the 

 ocean. This pelagic and insolated station is attained by the 

 Radiolaria through the evolution of calymmal structures in which 

 nitrogen is in all probability abundantly present. These swarms 

 of inert mucilaginous Radiolarian capsules and colonies are therefore 

 in every way suitable media for the nutrition of the zooxanthellae. 

 Attracted in all probability chemotactically by the nitrogenous 

 stores in the mucilage, the zooxanthellae enter the ectoplasm and 

 then divide and assimilate. Protected by their cellulose envelope, 

 they can at first resist the digestive enzymes of their host ; 

 ultimately, however, their nucleus becomes degenerate, and with 

 this change the protective wall, whose formation it governs, becomes 

 weakened. In this way some of the daughter-cells of the primary 

 zooxanthellae become food for their host (Famintzin). The 

 Radiolarian, which in its early stages fed on Peridinians, Infusoria, 

 and small Crustacea, ceases to ingest solid food and relies upon the 

 reserves it has accumulated or upon the secondary xanthellae for 

 its supplies. Meanwhile, its nitrogenous metabolism, and possibly 

 its intramolecular respiration, is maintained by the xanthellae, 

 which are removing the waste nitrogenous substances. In confirma- 



9 



