yiii THE VOYAGE OF H.M.S. CHALLENGER. 



The colonial Eadiolaria were described as early as the year 1834 by Meyen, the first investigator 

 of the class, under the name Sphcerozoum, and, as Palmellaria, compared with the gelatinous colonies 

 of the Nostochineae. The first accurate observations upon their structure were, however, made in 

 1851 by Huxley, who described examples of all three families under the name Thalassicolla 

 punctata. More extended, however, were the investigations of Johannes Miiller, who in his funda- 

 mental work (1858) divided the whole class Eadiolaria into Solitaria and Polyzoa. The Eadiolaria 

 solitaria he divided into Thalassicolla, Polycystina and Acanthometra, the Eadwlarm polyzoa into 

 Sphaerozoa (without a shell) and Collosphaera (with a shell). The most accurate delineation of the 

 Polycyttaria was given by Hertwig in his beautiful memoir, Zur Histologie der Eadiolarien (1876). 

 Quite recently, however (1886), since the completion of my manuscript upon the Challenger 

 Eadiolaria, a very complete Monograph of the Polycyttaria has appeared by Karl Brandt, Die 

 colonie-bildenden Eadiolarien (Sphaerozoen) des Golfes von Neapel und der angrenzenden Meeres- 

 Abschnitte (276 pp., 8 pis., Berlin). It contains in particular most valuable contributions to the 

 physiology and histology. 



15. The Central Capsule and Extracapsulum. The special peculiarity of the 

 unicellular Radiolarian organism, by which it is clearly distinguished from all other 

 Ehizopoda (and indeed from most other Protista), is its differentiation into two separate 

 chief constituents, the central capsule and extracapsulum, and the formation of a special 

 membrane which separates them. This, the capsule-membrane, is not to be compared 

 with an ordinary cell-membrane, as an external layer, but rather to be regarded as an 

 internal differentiated product. The extracapsulum or external (cortical) portion of the 

 body is in most Radiolaria more voluminous than the central capsule or inner (medullary) 

 portion. The exoplasm of the former (the cortical or extracapsular protoplasm) is 

 emphatically different from the endoplasm of the latter (the medullary or intracapsular 

 protoplasm). Besides the most important vital processes are distributed by division 

 of labour so completely between them that they appear most distinctly co-ordinated. 

 The central capsule is on the one hand the general central organ of the " cell-soul " for 

 the discharge of its sensory and motor functions (comparable to a ganglion-cell), on the 

 other hand the special organ of reproduction (sporangium). The extracapsulum, also, is 

 not less significant, since on the one hand its calymma acts as a protecting envelope to 

 the central capsule, as a support to the pseudopodia, and a foundation for the skeleton or 

 a matrix for the development of the shell, and on the other hand its pseudopodia are of 

 the utmost importance as peripheral organs of movement and sensation as well as of 

 nutrition and respiration. The central capsule and the extracapsulum are therefore to be 

 regarded both morphologically and physiologically as the two characteristic co-ordinated 

 principal parts of the unicellular Eadiolarian organism. 



In most of the more modern delineations of the Eadiolaria the central capsule is regarded as 

 the " cell proper " and its membrane as the " cell-wall." The following facts are opposed to the 

 correctness of this interpretation : 1. In most Eadiolaria the exoplasm is clearly different from 



