260 A. EARLAND OX RADIOLARIA. 



(2) The enclosed endoplasm or intra-capsular protoplasm ; (o) 

 The nucleus. In addition to these constant elements it also often 

 but not invariably contains other features, such as : — 



(4) An internal or intra-capsular skeleton. 



(5) Intra-capsular vacuoles or alveoli. 



(6) Fat granules or oil granules. 



(7) Crystals of differing composition. 



(8) Pigment granules. 



(9) Amyloid concretions. 



(10) Xanthell^e or "yellow cells." 



The Nucleus. 



The nucleus of a Kadiolarian is a large true cell nucleus, placed 

 either in the centre of the capsule, as in most Peripylea, or ex- 

 centrically, as in most other Radiolaria. At first solid, it usually 

 differentiates later into an outer dense nuclear membrane, and an 

 inner softer or fluid content, either with a single nucleolus or a 

 variable number of nuclei. This division of the nucleus into 

 nucleoli takes place at different periods, so that the Radiolaria 

 may be divided into precocious and serotinous. The bulk of 

 the Radiolaria belong to the latter or serotinous division, as the 

 division of the nucleus is deferred until shortly or immediately 

 before the process of spore formation. The nucleus then breaks 

 up rapidly into numerous small nuclei, sometimes thousands in 

 number, and each of these either becomes in itself the nucleus of a 

 swarm spore, or by repeated division gives rise to a group of 

 spore nuclei. But all the social or colonial Radiolarians (Poly- 

 cyttaria) and most of the Acantharia are precocious, for in them 

 the nucleus divides early in the life history of the cell, which is 

 therefore multi-nuclear during the greater part of its existence. 



The Endoplasm. 



The intra-capsular protoplasm or endoplasm is originally, and 

 in the earliest stages, the only important content of the capsule 

 except the nucleus. In this condition, which in some of the 

 smaller Radiolaria persists for a long period, the endoplasm 

 appears as a homogeneous, colourless, and finely granular mass, 

 having no definite structure and in particular no fibrillar net- 



