98 THE RADIOLARIA 



a cellulose wall, two chloroplasts marked by diatomin or an allied 

 pigment, a pyrenoid, starch of hollow and solid varieties, and a 

 nucleus. During the life of their host the zooxanthellae multiply 

 by transverse fission. After its death they pass into a "palmella 

 state " characterised by a mucilaginous jelly, and from this they 

 often escape as active biflagellated zoospores. 



Such zooxanthellae are frequent though not constantly present 

 in Thalassicolla. In T. nudeata they may be plentiful, scarce, or 

 absent. In most species they occur unfailingly ; sometimes in the 

 outermost jelly, sometimes in radial masses throughout the calymma, 

 or aggregated round the capsule, but never within it. The adapta- 

 tion of their host to surface life meets the requirements of the 

 yellow cells for light, oxygen, and no doubt other unascertained 

 demands, with the result that the association has been regarded as 

 one of mutual advantage, as a case of symbiosis. 



The more recent work of Famintzin (13) has, however, tended 

 to diminish the importance of the part which, according to Brandt, 

 is played by the abundant starch of the yellow cells in nourishing 

 their host. According to the later writer, the nutrition of Thalassi- 

 colla is mainly derived from ingested organisms, and is only aided 

 by the yellow cells in as far as these bodies are digested by the 

 ectoplasm. It is probable, though exact demonstration is as yet 

 wanting, that in some diffusible state exchange of material does take 

 place from zooxanthella to host without involving the death and 

 digestion of the former. Such a relation, however, does not explain 

 the presence of the yellow cells in Radiolaria. 1 



Respiration. The researches of Vernon (22) have shown that 

 gelatinous or mucilaginous pelagic animals have a high rate of 

 destructive metabolism, and that the amount of oxygen absorbed 

 per unit of dry body -weight is further increased in the smaller 

 animals as compared with the larger members of the same group, 

 and in those of warmer seas as against their cold-water relatives. 

 The maximum relative absorption of oxygen amongst inverte- 

 brate planktonic animals is reached, according to Vernon, in the 

 Radiolaria. Collozoum, a near ally of Thalassicolla, has the highest 

 coefficient of all invertebrates, equivalent to forty times that of 

 the frog ; and although it is desirable to have further evidence 

 before accepting this startlingly high figure, yet the evidence of other 

 pelagic forms points unmistakably to a very large consumption of 

 oxygen. 



The recent work of Putter (43) has emphasised the singular 

 nature of Protozoon respiration. It has long been known that 

 many of these organisms can live for a time as anaerobes, and it 

 now appears that intramolecular respiration obtains in a great 

 number of cases and to an unexpected extent. Fresh energy is set 

 1 For a discussion of the origin of the association see below, p. 129. 



