THE RADIOLARIA 131 



regard the skeleton as a response to the varying media, stresses, 

 and strains that fall upon the cytoplasm from within and from 

 without, that then its utilitarian character Avill be more completely 

 recognised, and its variety found to be explicable in terms of com- 

 position, mode of deposition, and the need of response to widely 

 varying combinations of stimuli that occur in the apparently mono- 

 tonous sea. Two very different substances compose the greater 

 part, and probably the whole, of these skeletal structures. In the 

 Spumellaria and Nassellaria pure silica is present ; in the Phaeodaria 

 the silex is mixed with organic substance ; but in the Acantharia 

 a substance is present which, from the time when it was first 

 described by Johannes Miiller to the present, has given rise to 

 differences of interpretation. Miiller, relying on the indestructible 

 nature of the Acantharian skeleton when heated, regarded it as 

 siliceous. Haeckel found that it was apparently destroyed by heat, 

 and regarded it in the main as an organic horny substance which 

 he called acanthin. Schewiakoff (33) tested its properties and 

 attempted a quantitative analysis, the result of which went to show 

 that the so-called acanthin was a complex silicate. Quite recently 

 Biitschli (39) has rein vesti gated the skeleton of Antarctic and of 

 fiome Mediterranean Acantharia, and has proved that in these 

 cases it is composed of strontium sulphate. 



The diverse forms of Radiolarian skeletons are largely founded 

 upon developments of scattered aciculate and tetrahedral spicules. 

 Dreyer has indeed attempted to trace the evolution of the skeleton 

 (1) in the Acantharia to an axopodial type derived from the 

 hardening of the axis that runs down the peculiar radiating 

 pseudopodia of this sub-class ; and (2) in other Radiolaria to the 

 modifications of a tetrad spicule, which in turn he traces to the 

 deposition of silica at the intersecting planes of adjacent cyto- 

 plasmic vacuoles or alveoles ; but the absence of a knowledge of 

 the development of the skeleton rendered this attempt suggestive 

 rather than convincing, and there are many forms of skeleton 

 which it is difficult to assign to any conceivable modification of the 

 tetrahedral type. In the present state of our knowledge it must 

 be admitted that the vacuolated cytoplasm has the power of 

 depositing its silica in the form of perforate or imperforate shells, 

 plates, and processes, so that in addition to the spicules there is 

 often a great development of siliceous matter, the form of which 

 cannot be referred to the alveolar structure of protoplasm. 



In form as in composition the skeleton of the Acantharia is 

 sharply marked off from that of other Radiolaria. With few 

 exceptions, it consists of twenty rods united in various ways : (1) by 

 opposition and also by adcentral processes ; (2) by fusion of all or 

 of opposite pairs, at the centre of the endoplasm. These radii are 

 .disposed so as to emerge from the spherical cytoplasm along five 



