:266 A. EAKLAXD ON RADIOLARIA. 



substance. In shape the individual phseodella is usually 

 spherical, sometimes ellipsoidal or lenticular. Their size varies 

 from very large specimens, '04: mm. in diameter, to very small, 

 •001 mm. or less. Perhaps an average size would be "OlS. 

 Haeckel, who first described the phfeodellae in 1862, stated that 

 a great part of them were true cells, composed of a nucleus 

 and protoplasm and enveloped by a membrane. Many of the 

 preparations mounted on board the Challenger confirm the 

 cellular nature of these organisms, but others, made at the same 

 time and by the same method, give no sign of nucleated cells, so 

 that their nature is still problematical. 



The physiological value of this curious feature is also a matter 

 of conjecture. That it must be of the highest importance for 

 some vital function can be conjectured from its size and 

 constant presence, while the fact that it is located round the 

 astropyle or main opening of the central capsule has led 

 to the supposition that it is in some way connected with the 

 nutrition and metastasis of the Phaeodaria. In this connection 

 it may be mentioned that the xanthellpe or yellow algoid cells, 

 which have been found living symbiotically in the bodies of many 

 Radiolaria of the other three legions, and which probably play 

 a considerable part in the life history of the organisms, are 

 apparently absent in all the Phieodaria. Nor does it seem 

 possible that they could exist there, for most Radiolaria cf this 

 legion are deep-sea forms, living in icy cold and eternal darkness. 

 Hence it has been surmised that their place in the Phaeodaria is 

 supplied by the phseodellse, which may possibly be some lower 

 form of algal life, capable of evolving oxygen under the influence 

 of the phosphorescence of deep-sea animals. 



The xanthell?e, which are frequently but not invariably found 

 in the extra-capsulum, will be described later in connection with 

 the life history of the Radiolaria. 



The Exoplasm or Extra-capsular Protoplasm. 



Although there is a constant interchange between the intra- 

 capsular and extra-capsular protoplasm through the openings in 

 the capsule membrane, the two portions of sarcode show certain 

 fixed and characteristic differences, due to the physiological 

 division of labour between the central and peripheral parts of the 



