THE RADIOLARIA 127 



refractive amyloid substance around a clear centre. In the Collodaria 

 they vary from '01.5 to '025 mm. in diameter ; in the Sphaerellaria 

 from '005 to '01 mm. In the Nassellaria the zooxanthellae are 

 very small in some Cyrtoidea (Eucecryphalus) ; very large in othei 

 (Eucyrtidium, Itictyopodiwn). A cellulose wall is present and en- 

 closes cytoplasm which contains two chromatophores impregnated 

 by chlorophyll and diatornin. In addition to the scattered hollow 

 vesicular, singly refractive structures that react to iodine by a 

 violet or bluish-violet tint, other doubly refractory granules occur, 

 and these are unaffected by iodine. After the death of the ecto- 

 plasm in which these zooxanthellae live, they pass into a palmella 

 stage and issue as bifiagellated organisms upon a free stage. The 

 structure and life-history of these zooxantliellae prove that they 

 are organisms living in association with Radiolaria, but it is not 

 possible to assign them to their true systematic position. Most 

 authors, following Biitschli, have placed them in the Crypto- 

 monadinae, a small heterogeneous group of simple algae ; but, as 

 Schaudinn has pointed out in his work on the zooxanthellae of 

 Trichosphaerium (42), it is also possible that these organisms have 

 quite other affinities. Brandt (10a) and Klebs (46) have drawn 

 attention to the similarity between the flagellated stage of the 

 xanthellae and the Peridinian Exuviaella marina. Further investiga- 

 tion of the behaviour of these yellow cells is necessary before their 

 position can be accurately defined. 



Yellow Cells of Acantharia. The xanthellae of the Acantharia 

 differ in many ways from those of other Radiolaria. They are 

 mainly intracapsular, and always naked cells. In some families 

 they assume a spherical form, in others an irregular amoeboid shape. 

 These cells pass by easy transitions to mere heaps of pigment 

 granules. When numerous they vary in size from *006 to '008 

 mm. When few they attain a much larger size, '015 to "03 mm. 

 The latter, which are found in Acanthoniidae, Lithopteridae, and 

 Amphilonchidae, are probably the largest zooxanthellae known. In 

 Acanthonia tetracopa and other members of the same family, besides 

 the usual intracapsular mass of zooxanthellae, a few occur now and 

 then in the extracapsulum. In Dorataspis and Actinomma large 

 amoeboid zooxanthellae occur regularly in this position. These 

 structures are almost constant in Acantharia, but they are absent 

 in young specimens and in the few species taken in deep water. 



The observations of Brandt (10a) on the finer structure of the 

 Acantharian zooxanthellae suggest that they have acquired a much 

 closer association with these Radiolaria than have those of Spumellaria 

 with their host, and that the older view of their nature was nearer to 

 the true significance of the association than the modern one that 

 regards the zooxanthellae as merely immigrant algae. Haeckel and 

 Hertwig regarded them as pigment cells formed by the segregation 



