THE SARCODINA 253 



stages of other Radiolarians have penetrated into the calymma, 

 and live there symbiotically a supposition which is certainly in 

 need of further proof before it can be accepted. 



3. The most external layer of the body is a protoplasmic envelope 

 from which the pseudopodia radiate. In Radiolaria, speaking 

 generally, the pseudopodia are straight, slender, and filamentous, 

 composed of motile protoplasm entirely (" myxopodia ") ; but in 

 Acantharia some of the pseudopodia are, like those of Heliozoa, 

 axopodia supported by stiff axial rods of organic substance, which 

 originate deep within the central capsule and pass through the 

 calymma along the axis of the pseudopodium, but without reaching 

 as far as its distal extremity. In some Acantharia (Acanthometrida) 

 are found also peculiar modifications of the bases of certain of the 

 pseudopodia in the form of groups of rod-like bodies, " myonemes " 

 or " myophrisks " (Fig. 105, my.), clustered round each of the 

 spicules of the skeleton. As their name implies, the myonemes are 

 contractile elements which, by their contraction or expansion, alter 

 the hydrostatic balance of the organism, and enable it to rise or 

 sink in the water. According to Moroff and Stiasny, the myonemes 

 are formed in the interior of the central capsule, and are derived 

 from nuclei. 



In a certain number of Radiolaria a skeleton is absent altogether. 

 The Acantharia have a skeleton composed of a substance which 

 was formerly supposed to be of organic nature, and was termed 

 acanthin by Haeckel, but which consists of strontium sulphate 

 -according to Butschli (310). In other Radiolaria the skeleton, 

 when present, is siliceous. In Acantharia the skeleton invades the 

 intracapsular region, and consists typically of a system of twenty 

 spines or spicules radiating from the centre of the body (Fig. 105). 

 It is a simple and enticing view to regard such a skeleton as origin- 

 ating phylogenetically from a modification of the axis of pseudo- 

 podia. Union of outgrowths from radially-directed spicules gives 

 rise to a lattice-work forming a spherical perforated shell, and as 

 the animal grows in size several such concentric spheres may be 

 formed, one within the other, supported by radial bars which 

 represent the original radiating spicules (Fig. 107). In Radiolaria 

 other than Acantharia the skeleton is usually entirely extracapsular, 

 and exhibits a variety of form and structure which cannot be dis- 

 cussed further here. In some of the Tripylara foreign bodies are 

 utilized for building up the skeleton, either to form the basis 

 of spines secreted by the animal or to construct a coat of armour 

 on the exterior of the body (Borgert). 



Life-History. Reproduction of the Radiolaria is effected in 

 some instances by binary fission namely, in those forms in which 

 <a skeleton is lacking or consists of loose spicules. The nucleus 



