GENERAL INTRODUCTION. 



ANATOMICAL SECTION. 



A SKETCH OF OUR KNOWLEDGE OF THE ORGANISATION OF THE KADIOLARIA 



IN THE YEAR 1884. 



Chapter I.— THE UNICELLULAR ORGANISM. 



(§§ 1-50.) 



1. Definition of the Radiolaria. — Radiolaria are marine Rhizopoda, whose 

 unicellular body always consists of two main portions, separated by a membrane ; an 

 inner Central capsule (with one or more nuclei) and an Extracapsulum (the external 

 calymma, which has no nucleus, and the pseudopodia) ; the endoplasm of the former 

 and the esoplasm of the latter are connected by openings in the capsule -membrane. 

 The central capsule is partly the general central organ of the Radiolarian cell, partly the 

 special organ of reproduction, since its intracapsular protoplasm, along with the nuclei 

 embedded in it, serves for the formation of flagellate spores. The extracapsulum is 

 partly the general organ for intercourse with the outer world (by means of the 

 pseudopodia), partly the special organ of protection (calymma) and nutrition (sarco- 

 matrix). The majority of Radiolaria develop also a skeleton for support and protection,^ 

 which presents the utmost variety of form, and is generally composed of silica, sometimes 

 of an organic substance (acanthin). The Radiolarian ceU usually leads an isolated 

 existence (Monozoa vel Monocyttaria) ; only in a smaU minority (of one legion) are the 

 unicellular organisms united in colonies or ccenobia (Polyzoa vel Polycyttaria). 



The extent of the Eadiolaria, as limited by the above definition, which I have made as 

 compact as possible, differs in several important respects from that allowed to the group by all 

 previous diagnoses. The shortest expression of its scope might perhaps be : — Ehizopoda with 

 central capsule and calymma; for the most important character of the Eadiolaria, and that by 

 which they are distinguished from aU other Ehizopoda, is the differentiation of the unicellular body 

 into two principal parts of equal importance and their separation by a constant capsule-membrane. 



2. The Tivo Subclasses of the Eadiolaria. — The systematic catalogue of the 

 Radiolaria, which forms the second part of this Report, and is brought up to the year 



(ZOOL. CHALL. EXP. — PART XL. — 1887.) Rr a 



