Moreover, the radiolarian skeletons are of quite extraordinary delicacy and complexity, in spite of their minuteness and the comparative simplicity of the ãunicellularã organisms within which they grow; and these complex conformations have a wonderful and unusual appearance of geometric regularity. All these general conôÙsiôÙdeôÙraôÙtions seem such as to prepare us for the special need of some physical hypothesis of causation. The little skeletal fabrics remind us of such objects as snow-crystals (themselves almost endless in their diversity), rather than of a collection of distinct animals, constructed in apparent accordance with functional needs, and distributed in accordance with their fitness for particular situations. Nevertheless great efforts have been made of recent years to attach ãa biological meaningã to these elaborate structures; and ãto justify the hope that in time the utilitarian character [of the skeleton] will be more completely recognised64.ã
In the majority of cases, the skeleton of the Radiolaria is composed, like that of so many sponges, of silica; in one large family, the Acantharia (and perhaps in some others), it is composed, in great part at least, of a very unusual constituent, namely strontium sulphate65. There is no fundamental or important morphological character in which the shells formed of these two constituents differ from one another; and in no case can the chemical properties of these inorganic materials be said to influence the form of the complex skeleton or shell, save only in this general way that, by their rigidity and toughness, they may give rise to a fabric far more delicate and slender than we find developed among calcareous organisms.
A slight exception to this rule is found in the presence of true crystals, which occur within the central capsules of certain {459} Radiolaria, for instance the genus Collosphaera66. Johannes Mû¥ller (whose knowledge and insight never fail to astonish us) remarked that these were identical in form with crystals of celestine, a sulphate of strontium and barium; and Bû¥tschliãs discovery of sulphates of strontium and of barium in kindred forms render it all but certain that they are actually true crystals of celestine67.
In its typical form, the Radiolarian body consists of a spherical mass of protoplasm, around which, and separated from it by some sort of porous ãcapsule,ã lies a frothy mass, composed of protoplasm honeycombed into a multitude of alveoli or vacuoles, filled with a fluid which can scarcely differ much from sea-water68. According to their surface-tension conditions, these vacuoles may appear more or less isolated and spherical, or joining together in a ãfrothã