RADIOLARIA. 



145 



structures. Moreover, among those types in which a skeleton is 

 developed, there are great differences as regards both the chemical 

 composition and the structure of this. As regards the first of these 

 points, the skeleton is never calcareous, but presents itself under one 

 or other of the following three modifications. In the first place, the 

 skeleton may be composed of pure silica. This is the case in the 

 Polycysti?ia (the Spumellaria and Nassellaria of Haeckel), which 

 constitute the largest and most typical group of Radiolarians. In 

 another series of forms the skeleton is a compound of silica with 

 some organic compound, or is a " silicate of carbon " (Haeckel). 

 This occurs in the entire group of the Phceodaria, except in Dicty- 

 ocha and its allies. Lastly, in a third series of forms the skeleton 

 consists of a peculiar organic substance ("acanthin") allied to 

 chitine or horn. The forms in which the skeleton is of this nature 

 are grouped together by Haeckel under the name of Acantharia. 



As regards the form of the skeleton, the most usual type is that of 

 a latticed shell (fig. 45), enclosing the central capsule. In certain 

 Radiolarians, however, the skeleton consists of radially or tangen- 



■ m* 



Fig. 45. — Skeletons of Polycystina. a, Stylodictya multispina ; b, Podocyrtis Schomburgii ; 

 c, Eitcyrtidium lagena. (After Haeckel.) 



tially disposed spicules ; or it may have the form of a simple ring, or 

 of a " basal tripod with or without a loose tissue of trabecular " (Haec- 

 kel). In the Polycystina {Spumellaria and Nassellaria), the siliceous 

 bars of the skeleton are invariably solid, whereas in the Phceodaria 

 the parts of the skeleton are always hollow. 



As regards their classification, Professor Haeckel raises the Radio- 

 larians to the rank of a distinct class of the Protozoa, and he divides 

 them into the following four legions or sub-classes : — 



1. Spumellaria. — Capsular membrane perforated by innumerable fine 

 pores. Fundamental form originally spherical. Skeleton siliceous, or 

 vol. I. K 



