Fig. 62.—A Group of Radiolarian Shells, magnified
Some of the radiolarians live at or near the surface of the ocean, while others thrive only at the bottom. The former, in some cases, appear to avoid the light, rising to the surface after sunset; and it is supposed that the phosphorescence of the sea is due in part to the presence of these animals. The latter may be obtained from all depths, down to several thousand fathoms.
The beauty of the radiolarians as a class lies in the wonderful shells that protect the great majority of them. These shells are composed not of carbonate of lime, as is the case with foraminifers, but of silex or silica, a substance that is not acted on by the strongest mineral acids. They are of the most exquisite shapes, and exhibit a great variety of forms. Some resemble beautifully sculptured spheres, boxes, bells, cups, &c.; while others may be likened to baskets of various ornamental design. In every case the siliceous framework consists of a number of clusters of radiating rods, all united by slender intertwining threads.
It is not all the radiolarians, however, that produce these beautiful siliceous shells. A few have no skeleton of any kind, while others are supported by a framework composed of a horny material, but yet transparent and glassy in appearance.
The sizes of the shells vary from about one five-hundredth to one half of an inch; but, of course, the larger shells are those of colonies of radiolarians, and not of single individuals, just as we observed was the case with the foraminifers.
Those in search of radiolaria for examination and study should, whenever possible, obtain small quantities of the dredgings from deep water. Material brought up by the trawl will often afford specimens; but, failing these sources of supply, the muddy deposit from deep niches between the rocks at low-water mark will often provide a very interesting variety.
Place the mud in a glass vessel, and pour on it some nitric acid (aqua-fortis). This will soon dissolve all calcareous matter present, and also destroy any organic material. A process of very careful washing is now necessary. Fill up the vessel with water, and allow some time for sedimentary matter to settle. Now decant off the greater part of the water, and repeat the process several times. By this means we get rid of the greater part of the organic material, as well as of the mineral matter that has been attacked by the acid; and if we examine the final sediment under the microscope, preferably in a drop of water, and covered with a cover-glass, any radiolarians present will soon reveal themselves.
It is often possible to obtain radiolarian shells, as well as other siliceous skeletons, through the agency of certain marine animals. The bivalve molluscs, for example, feed almost entirely on microscopic organisms; and, by removing such animals from their shells, and then destroying their bodies with aqua-fortis, we may frequently obtain a sediment composed partly of the skeletons referred to.
There remains one other class of protozoons to be considered, viz. the Infusorians—the highest class of the sub-kingdom. In this group we observe a distinct advance in organisation; for, in the first place, the infusorians are enclosed in a firm cuticle or skin, which forms an almost complete protective layer. Within this is a layer of moderately firm protoplasm, containing one or more cavities that contract at intervals like a heart. Then, in the interior, there is a mass of softer material with cavities filled with fluid, two solid bodies, and numerous granules.