126 



THE RADIOLARIA 



These complex mitotic phenomena offer many peculiarities, some 

 of which are discussed by Borgert. The absence of a spindle and 

 of centrosomes, the double splitting, great number and variety of 

 the chromosomes, the peculiar twisting of the nucleus and equatorial 

 plate, and the two spirem stages render this form of karyokinesis 

 unique ; and in spite of the labour which has been bestowed upon 

 its analysis, several points, such as the origin and fate of the equa- 

 torial membrane and the formation of the daughter nuclei, are still 

 obscure. 



Yellow Cells. Zooxanthellae occur commonly in the ectoplasm of 

 Spumellaria and Nassellaria ; in the endoplasm of Acantharia ; and 



chr. 



FIG. 16. 



land 2, two specimens of Collozoum inerme, showing zooxanthellae (Z) in the ectoplasm, xlOO. 

 3, 4, and 5 are magnified views of a single xanthella, showing its escape as a biflagellated 

 organism from the cyst which it forms during the palmella state ( x 330). K, the nucleus ; 

 chr, the two chroma tophores ; the inclusions are hollow, amyloid grains. (After Brandt.) 



are absent from the Phaeodaria. Their occurrence is facultative 

 and not absolutely constant. They are very abundant in orders 

 with a well-developed calymma such as the Collodaria (both mono- 

 zoic and polyzoic), less so in the Sphaerellaria, and in Physematiidae 

 with no extracapsular vacuoles, and are absent in the Discoidea. 

 Similarly, zooxanthellae increase in number with the increase of size 

 of the animal or coenobium in which they occur. Young colonies of 

 Collozoum up to 50 or 100 members contain few or no zooxanthellae, 

 older ones become impregnated with them. 



The zooxanthellae of the Spumellaria are similar in structure 

 and behaviour to those of Thalassicolla (pp. 97-8). They are usually 

 spherical organisms with a single apparently homogeneous nucleus, 

 capable of assimilating carbon and of forming sheaths of a singly 



