ANIMALCULES. 



717 



derived from the surrounding water, takes the form of rods or spikes which 

 pierce the soft body, and also often form some kind of protective framework 

 around it. Another peculiarity of the Radiolaria is that the soft body is 

 divided into an inner portion, containing the nucleus and enclosed in a 

 gelatinous naembrane called a capsule, and an outer portion which surrounds 

 the other and is itself enclosed in a gelatinous envelope. The inner capsule 

 is perforated so that the inner protoplasm can pass outward, while the outer 

 protoplasm sends out radiating threads in all directions through the number- 

 less fine perforations of its envelope. 



A vast number of exquisite forms assumed by the skeletons of the Radio- 

 laria were found in the dredgings made by the Challenger expedition. Some 

 of these skeletons consist only of spines radiating from the centre which, 

 however, are often beautifully sculptured or branched ; in one form the 

 number of the spines is always twenty, and these are arranged with absolute 

 regularity at definite angles to each other. The whole animal often has the 

 appearance of a spiny ball (Fig. 5, A). In other cases the hard rods, which 



C, Actinomnia. 



pig_ 5^ — Skeletons of Radiolaria. 



B, Lithomelissa. ^, Acanthometra. 



form the constituent elements of all these skeletons, may be arranged so as 

 to form a framework or network outside the protoplasm with hexagonal 

 apertures. Such a framework may be more or less round for a time, a,nd 

 then a second framework may be added below the first with a wide opening 

 at its base— the whole having the appearance of a helmet ornamented at 

 definite points with a few of the characteristic spines that pierce the inner 

 protoplasm (B). Sometimes a whole series of fenestrated balls is produced, 

 one outside the other (0), recalling the elaborate cirved ivory spheres of the 

 Chinese. In the illustration, some of the outer spheres have been broken 

 through to show the inner structure. Such successive spheres are firmly 

 united together by means of cross rods or of the characteristic spines. 



The Radiolaria are all found in salt water, but, unlike the Forammifera, 

 float near the surface. Their skeletons form an important element in the 

 rocks of Sicily and Greece; and the " Barbadoes earth,'; which is used for 

 polishing, is mainly composed of the flinty skeletons of Radiolarians. Although 



