Iviii THE VOYAGE OF H.M.S. CHALLENGER. 



presence has not been certainly demonstrated. The extracapsular Xanthellae are found 

 most abundantly in the Collodaria, both in the monozootic Thalassicollida and in 

 the polyzootic Sphserozoida. They occur in smaller numbers in the Sphserellaria, 

 and in many divisions of the latter they seem to be entirely absent. Also it sometimes 

 happens that, though present in large numbers in some SPUMELLARIA, they are entirely 

 absent in others nearly related to them; indeed, this has also been observed in the case 

 of different individuals of the same species. This fact alone is sufficient to show that 

 the Xanthellse are not an integral part of the Eadiolarian organism (as was formerly 

 believed) but parasites or more correctly symbiontes, which live as inhabitants of the 

 calymma. More recent investigations have shown, that besides the yellow pigment- 

 grains they contain starch or an amyloid substance, that is to say, vegetable reserve 

 materials, that their thin envelope contains cellulose, and that their yellow colouring- 

 matter resembles chlorophyll and is related to that of the Diatomacese (" Diatomin "). 

 Hence they are now generally regarded as unicellular Algse, nearly related to those 

 which occur as symbiontes in other marine animals (Exuviella, &c.). The starch, 

 which they develop with the formation of oxygen, may serve as nutriment to the 

 Radiolaria, while the carbonic acid yielded by the latter is also beneficial to the 

 Xanthellse. The form of the Xanthellae is usually spherical and elliptical, often also 

 sphseroidal or discoidal. Their diameter is usually between O'OOS and 0'012 mm., 

 rarely more or less. The differences exhibited by Xanthellae which live in different 

 groups of Radiolaria demand further investigation, which will perhaps lead to the 

 establishment of several species of the genus Zooxanthella. At present Zooxanthella 

 extracapsularis, in the calymma of SPUMELLARIA and NASSELLARIA, may be clearly dis- 

 tinguished from Zooxanthella intracapsularis, in the central capsule of the ACANTHARIA. 



The " yellow cells " were first described in 1851 by Huxley, in the Collodaria, and after- 

 wards by J. Mliller (1858) in many SPUMELLARIA and NASSELLARIA. In my Monograph (1862, 

 pp. 8487) I gave a detailed account of their structure and increase by division, and laid special 

 emphasis on the fact that they are the only elements in the Radiolarian organism which "are 

 undoubtedly cells in the strict histological sense of the word." Afterwards, in my Beitriige zur 

 Plastiden-Theorie, I showed the constant presence of " starch in the yellow cells of the Radiolaria " 

 (1870, L. N. 21). Shortly afterwards Cienkowski observed that the yellow cells live independently 

 and reproduce themselves after the death of the Radiolaria, and in consequence first put forth the 

 hypothesis that they do not belong to the Radiolarian organism, but that they are unicellular 

 Algae parasitic upon it (1871, L. N. 22). This view was ten years later more fully established 

 by Karl Brandt, and elucidated by comparison with the symbiosis of the gonidia of Algae, and 

 the hyphse of Fungi in the formation of Lichens, which had in the meantime become known 

 (1881, L. N. 38). Brandt gave this unicellular yellow Alga the name Zooxanthella 

 nutricola, and afterwards gave fuller details regarding its remarkable vital relations (L. N. 39), 

 Patrick Geddes, who named it PMlozoon, supplemented this account and showed experimentally 

 that it gives off oxygen under the influence of sun-light (1882, L. N. 42, 43). In consequence 



