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



The free astropodia, which pass outwards from the sarcodictyum into the water, are in 

 most Ph^odaria very numerous (PL 101, fig. 10). Since, however, only a few 

 species of this great legion have been observed in a living state, their pseudopodia 

 require further accurate examination. 



Chapter IV.— THE SKELETON. 



(§§ 101-140). 



101. TJie Significance of the Skeleton. — The skeleton of the Radiolaria is developed 

 in such exceedingly manifold and various shapes, and exhibits at the same time such 

 wonderful regularity and delicacy in its adjustments, that in both these respects the 

 present group of Protista excels all other classes of the organic world. For, in spite of 

 the fact that the Radiolarian organism always remains merely a single cell, it shows the 

 potentiality of the highest complexity to which the process of skeleton formation can be 

 brought by a single cell. All that has been brought to pass in this direction hj single 

 tissue-cells of animals and plants does not attain the extremely high stage of development 

 of the Radiolaria. Only ver}^ few Rhizopoda of this very rich and varied class fail to 

 exhibit the power of forming this firm supporting and protecting organ — indeed, only 

 ten of the seven hundred and thirty-nine genera which are enrolled in the list of the 

 Challenger collection, namely, six genera of Spumellaria (five Thalassicollida, Actissa, 

 Thalassolmnpe, Thalassojiila, Thalassicolla, Thalassophysa, PI. 1, and one genus of 

 Collozoida, Collozoum, PL 3), and in addition two genera of Nassellaria (the Nassellida, 

 Cystidium and Nassella, PL 91, fig. 1), and two genera of Ph^odaria (the Phaeodinida, 

 Phceocolla and Phwodina, PL 101, figs. 1. 2). These skeletonless forms of Radiolaria 

 are, however, of extreme interest, since they include the original stem-forms of the whole 

 class as well as of its four legions. All Radiolaria which form skeletons have originated 

 from soft and skeletonless stem-forms by adaptation, and that polyphyletically, for the 

 skeletal types of the four legions have been developed independently of each other (§ 108). 



102. The Chemical Peculiarities of the Skeleton. — The chemical composition of the 

 skeleton shows very marked variations in the different legions of the Radiolaria. The two 

 legions Spumellaria and Nassellaria (united formerly as " Polycystina ") form their 

 skeleton of pure silica (see note A, below) ; the legion PHiEODARiA of a silicate of carbon 

 (see note B), and the Acantharia of a peculiar organic substance — acanthin (see note C). 

 This explains the well-known fact that the deposits of fossil Radiolaria (or Polj^cystine 

 marls) are composed exclusively of the skeletons of Spumellaria and Nassellaria, those 

 of the Acantharia and Ph.eodaria being entirely absent (in the case of the last group, 

 however, exception must be made in favour of the Dictyochida, or those Ph^iiodaria 



