A. EARLAND OX RADIOLARIA. 269 



ill the protoplasm of the myxopodia, which never contain an axial 

 thread. 



The axopodia, on the other hand, are highly differentiated 

 pseudopodia, consisting of a firm radial thread or core, with a soft 

 covering of protoplasm. They penetrate the whole calymma in 

 a radial manner, projecting fixedly from its outer surface, and 

 are generally, if not always, produced inwards to the middle 

 of the central capsule, perforating the capsule membrane. They 

 <are known only in the second legion, the Acantharia, in which 

 they are widely, if not universally developed. The fine axial 

 thread may be of acant/mi, the chitinous material of which 

 the Acantharia construct their skeletons. The axopodia are 

 probably sensory organs, and are not retractile, while the 

 myxopodia are. 



The Skeleton. 



The Radiolaria have long been the delight of microscopists for 

 the regularity and delicacy of their shells, which are developed in 

 a wealth of variety far exceeding other forms of microscopic life. 

 Although the Radiolarian organism always remains an individual 

 cell, it shows the high complexity to which the process of skeleton 

 formation can be brought by a simple cell. Very few Radiolaria 

 are destitute of these skeletons, only ten of the seven hundred 

 and thirty-nine genei-a described by Haeckel in his Challenger 

 monograph being skeletonless. 8ix of these belong to the first 

 legion, the Spumellaria, and there are two skeletonless genera in 

 the third and fourth legions, the Nassellaria and the Ph^eodaria. 

 Though numerically insignificant, these skeletonless Radiolaria 

 are of the highest biological interest, as they probably represent 

 the more archaic and ancestral forms from which the whole 

 class has been evolved. 



The material of which the skeleton is composed varies in the 

 different legions. The first and third legions, the Spumellaria 

 and Nassellaria, have shells of pure silica. The second legion, 

 the Acantharia, use a peculiar organic substance, allied in its 

 composition to chitin, called acanthin ; while the fourth legion, 

 the Phseodaria (except one family, the Dictyochida), have 

 skeletons of an organic compound, which is apparently a silicate 

 of carbon. There are no Radiolaria with calcareous skeletons. 



JouRN. Q. M. C, Series II.— No. 46. 20 



