A. EARLAND ON RADIOLARIA. 273 



reaching a certain size the buds become separated from the 

 parent capsule, and are distributed through the sarcoplegma by 

 the currents of the exoph^sm. Here each bud develops into 

 a complete central capsule by the formation of a capsular 

 membrane. 



It has been observed by Hertwig and Karl Brandt that certain 

 of the social liadiolaria form two distinct kinds of spores, which 

 liave been called isospores and anisospores. The iso.spores are the 

 same asexual zoospores as have been observed in many Radiolaria, 

 but the anisospores, which are in two forms, female macrospores 

 and male microspores, are sexually differentiated, and probably 

 conjugate after their exit from the central capsule, thus producing 

 a new cell. As the same colony has been observed to produce 

 the sexual and asexual spores at different times, it is probable 

 that they alternate, and that we have here a true instance of 

 Alternation of Generations. 



The digestive and nutritive functions of the Radiolaria are 

 purely animal, and like all other animals they are compelled to 

 obtain their nutriment in a ready formed condition, having no 

 power to form it synthetically after the manner of plants. The 

 oxygen may be derived either from the surrounding water or 

 from the symbiotic xanthellse, which under the action of direct 

 sunlight evolve large quantities of oxygen. 



These xanthella? have already been mentioned as frequent, 

 but not invariable, enclosures in the Radiolarian body ; and as 

 they play a most important part in the functions of the animal, 

 and are also of the highest interest to the biologist, they call for 

 a detailed description. 



The xanthella^, or " yellow cells," of the Radiolaria were first 

 discovered by Huxley, who described them in 1851 in certain 

 species of Collodaria, the family in which they are now known 

 to occur most abundantly. In 1858 J. Miiller described their 

 occurrence in the extra-capsular protoplasm of many Spumellaria 

 and jSTassellaria, and also within the central capsule of certain 

 Acantharia. Haeckel, in 1862, gave a detailed description of 

 their structure and increase by division, proving that they were 

 true cells. At this time, and for several years subsequently, the 

 xanthellfe were believed to be integral parts of the Radiolarian 

 organism, and hence the Radiolaria were considered to be 

 unquestionably multicellular animals. In 1870 Haeckel proved 



