257 



Radiolaria. 



By Arthur Earland. 



(^Read November 17t7i, 1899.) 



Plates 15 and 16. 



The Radiolaria, which are of exclusively marine habitat, form 

 an order of the class Rhizopoda, and have in general the charac- 

 teristic organisation of that class, but they differ from all other 

 Rhizopoda in the fact that their unicellular body is divided by 

 a porous membrane, known as the capsule membrane, into two 

 clearly marked portions, a central capsule containing one or more 

 nuclei, which may be considered mainly as the organ of repro- 

 duction and sensation, and an extra-capsulum with a calymma, 

 or " jelly-veil," from which the pseudopodia diverge. The extra- 

 capsulum may be regarded mainly as the organ of nutrition and 

 motion. The sarcode of the two portions communicates by means 

 of openings in the capsule membrane. Nearly all Radiolaria 

 develop a skeleton for support and protection, and these skeletons 

 present a most wonderful variety of forms. In the first, third, 

 and a part of the fourth legions the skeletons are purely siliceous, 

 but in the second legion, the Acantharia, the skeleton is always 

 composed of an organic substance of a chitinous nature, known 

 as acanthin, while the majority of the fourth legion have a skeleton 

 composed of a silicate of carbon. Calcareous skeletons are unknown 

 in the Radiolaria. The Radiolarian cell in most instances leads 

 an isolated and separate existence (Monoeyttaria), but in a few 

 genera of the first legion, the Spumellaria, a number of individuals 

 combine to form a colony or ccenohium (Polycyttaria). 



It will be seen from the foregoing definition that the most 

 important character of the Radiolaria is the differentiation of the 

 unicellular body into two principal parts of equal importance, 

 and the separation of these two parts by a constant capsule 

 membrane. 



