REPORT ON THE RADIOLARIA. CXXxi 



striking instance of Thalassicolla sanguinolenta, which becomes so deformed by the inception of 

 numerous coccospheres and coccoliths, that I described it as a special genus under the name 

 Myxobrachia (compare pp. 23, 30 ; also L. N. 21, p. 519, Taf. xviii., and L. N. 33, p. 37). 



I . 



205. Symbiosis. Very many Radiolaria, but by no means all members of this 

 class, live in a definite commensal relation with yellow unicellular Algae of the group 

 Xanthellse. In the ACANTHARIA they live within the central capsule (Zooxanthella 

 intracapsularis, 76), in the SPUMELLARIA and NASSELLARIA, on the other hand, 

 within the calymma but outside the central capsule (Zooxanthella extracapsularis, 

 90) ; in the PH^ODARIA a special form of these symbiotic unicellular Algae appears 

 to inhabit the phaeodium in the extracapsulum, and to compose a considerable portion 

 of the phaeodellse (Zooxanthella phceodaris, 90, or better perhaps Zoochlorella 

 phdBodaris, 89). Undoubtedly this commensal life is in very many cases of the 

 greatest physiological significance for both the symbiontes, for the animal Radiolarian 

 cells furnish the inquiline Xanthellae not only with shelter and protection, but also with 

 carbon dioxide and other products of decomposition for their nutriment ; whilst on the 

 other hand the vegetable cells of the Xanthellse yield the Radiolarian host its most im- 

 portant supply of nutriment, protoplasm and starch, as well as oxygen for respiration. 

 Hence it is not only theoretically possible, but has been experimentally proved, that 

 Radiolaria which contain numerous Xanthellae can exist without extraneous nutriment 

 for a long period in closed vessels of filtered sea-water, kept exposed to the sunlight; 

 the two symbiontes furnish each other mutually with nourishment, and are physio- 

 logically supplementary to each other by reason of the opposite nature of their 

 metastasis. This symbiosis is not necessary, however, for the existence of the 

 Radiolaria ; for in many species the number of Xanthellae is very variable and in 

 many others they are entirely wanting. 



The symbiosis of the Eadiolaria and Xanthellse, or " yellow cells " ( 76, 90) was first discovered 

 by Cienkowski in 1871 (L. N". 22). Ten years later this important and often doubted fact was 

 established by extended observations and experiments almost simultaneously by Karl Brandt 

 (L. N. 38, 39) and Patrick Geddes (L. N. 42, 43). This commensal life may be compared with 

 that of the lichens, in which an organism with vegetable metastasis (the Algoid gonidia) and an 

 organism with animal metastasis (the Fungoid hyphae) are intimately united for mutual benefit. 

 But the symbiosis of the Xanthellae and Eadiolaria is not as in the lichens a phenomenon essential for 

 their development, but has more or less the character of an accidental association. The number of 

 the inquiline Xanthellas is so variable even in one and the same species of Eadiolaria, that they do 

 not appear to be exactly essential to its welfare ; and in many species they are entirely wanting. 

 Their significance is questionable in the case of those numerous deep-sea Eadiolaria which live in 

 complete darkness, and in which, therefore, the Xanthellae, even if present, could excrete no oxygen 

 on account of the want of light. Nevertheless it is possible that the phseodellae of the PILEODARIA 

 (usually green, olive, or brown in colour), which are true cells, represent vegetable symbioutes, 



