112 THE SEA SHORE 



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 radia- 

 ting 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, pre- 

 ferably 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 micro- 

 scopic 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 



