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



and Patrick Geddes (L. N. 42, 43). This arrangement was compared by Brandt to tlie 

 remarkable symbiosis of the Algoid gonidia and Fungoid hyphas in the organisation of 

 the Lichens, which had been recently discovered, and since he recognised the independent 

 nature of the yellow cells, as unicellular Algse, in all divisions of the Eadiolaria, he 

 founded for them the genus Zooxanthella. Geddes named them Philozoon, and showed 

 experimentally that they give out oxygen under the influence of sunlight (compare § 90). 

 The great physiological importance of the yellow cells in the metastasis of the Eadiolaria, 

 and, when they are developed in large quantities, in the economy of marine organisms 

 in general, has recently been insisted upon by Brandt (see § 205 and L. N. 52, 

 pp. 65-71, 86-94). 



The proof that the yellow cells do not belong to the Radiolarian organism itself, but 

 only live parasitically in it, was a necessary preliminarj^ to the very important step 

 which next took place in our knowledge of the organisation of the Eadiolaria. This 

 step consisted in the demonstration that the whole body of the Eadiolaria, like that of 

 all other Protista, is only a single cell. It was Eichard Hertwig who in two remarkable 

 works (L. N. 26, 33) firmly established this fundamental theorem of the unicellular 

 nature of the Eadiolaria. In his treatise on the histology of the Eadiolaria (L. N. 26, 

 1876) he published complete investigations into the structure and development of the 

 Sphserozoida and ThalassicoUida. Since he made use of the modern methods of 

 histological examination, and especially of staining fluids, which he was the first to 

 apply to the study of the Eadiolaria, he was able to show that no true cells (apart from 

 the parasitic yellow cells) are to be found in their bodies, but rather that all their 

 morphological components are to be regarded as difi'erentiated parts of a single true cell, 

 and in particular that the central capsule includes a genuine nucleus. 



A wider foundation for this important discovery and its applicability to aU divisions 

 of this extensive class, was given by Hertwig in a second work on the organisation of 

 the Eadiolaria (L. N. 33, 1879). Among the numerous discoveries by which this work 

 enriched the natural history of the Eadiolaria must be specially mentioned the recognition 

 of the fundamental difi"erences exhibited by the main divisions of the class in the structure 

 of their central capsule. Hertwig first observed that the capsular membrane is double 

 in the Ph^odaria but single in the other Eadiolaria (§ 56) ; the former he named " Tri- 

 PYLEA " because he discovered in then- capsular membrane a large, peculiarly constructed 

 main opening and two small accessor)^ openings. The Nassellaria, in which he found 

 a single porous area at the basal pole of the main axis, with a cone of pseudopodia rising 

 from it, he called on this account " Monopylea " ; whilst the other Eadiolaria, whose 

 capsular membrane is perforated on all sides with fine pores, were termed " Peripylea." 

 Besides the central capsule, Hertwig laid stress upon the significance of the gelatinous 

 envelope as a constant and important constituent of the body. He also devoted 

 attentive consideration to the morphology of the skeleton, and on the basis of certain 



