90. _The Extracapsular Xanthellæ._--Xanthellæ or Zooxanthellæ, symbiotic "yellow cells," are very commonly found in the extracapsulum of the Radiolaria, especially in many SPUMELLARIA and NASSELLARIA; whilst in the ACANTHARIA similar yellow cells usually only occur within the central capsule, and in the PHÆODARIA their {lviii}presence has not been certainly demonstrated. The extracapsular Xanthellæ are found most abundantly in the #Collodaria#, both in the monozootic Thalassicollida and in the polyzootic Sphærozoida. They occur in smaller numbers in the #Sphærellaria#, 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 Xanthellæ are not an integral part of the Radiolarian 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 Diatomaceæ ("Diatomin"). Hence they are now generally regarded as unicellular Algæ, 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 Xanthellæ. The form of the Xanthellæ is usually spherical and elliptical, often also sphæroidal or discoidal. Their diameter is usually between 0.008 and 0.012 mm., rarely more or less. The differences exhibited by Xanthellæ 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 distinguished from _Zooxanthella intracapsularis_, in the central capsule of the ACANTHARIA.