Cxlii THE VOYAGE OF H.M.S. CHALLENGER. 



contractiou of the central capsule is probably brought about by the myophanes, which 

 arise by differentiation of the endoplasm and hence may assume different forms in the 

 four legions. In the Spumellaeia, where numerous radial fibriUse run from the central 

 nucleus to the capsule membrane (§ 77), the endoplasm is probably driven out evenl}^ 

 through all the pores of the capsule membrane by their simultaneous contraction, and 

 hence the volume of the capsule is diminished in all directions. The Acanthakia 

 probably behave similarly, but are different, inasmuch as the number of their contractile 

 radial fibrillse is less, and special axial threads (§ 78) are already differentiated. In the 

 Nassellaria it is probal^le that owing to the contraction of the divergent myophane 

 fibrillje in the podoconus the vertical axis of the latter is shortened, the opercular rods 

 of the porochora are lifted, and the endoplasm driven out of its pores, so that the 

 volume of the nionaxon central capsule is diminished (§ 79). In the Ph^odaria the 

 same result is probably brought about by the contraction of the cortical myophane 

 iibriH^, which run meridionaUy along the inside of the capsule membrane from the 

 apical to the basal pole of the vertical main axis, where they are inserted into the 

 periphery of the astropyle ; since the volume of the capsule is diminished by their 

 contraction (their spheroidal figure becoming more nearly spherical) the endoplasm wiU 

 be driven out through the proboscis of the astropyle. Whilst these contractions of the 

 central capsule are largely due to differentiated muscle-like threads of endojalasm 

 (myophanes), this appears to be but rarely the case with the contractions of the 

 extracapsulum (e.(/., the myophriscs of the Ac an t h oni e tra, § 96). Most of the 

 phenomena of contraction which can be observed in the calymma and pseudopodia 

 depend upon exoplasmatic currents (§ 209). 



222. Protection. — Of the utmost importance, both for the ^jhysiology and for the 

 morphology of the Radiolaria are their manifold protective functions, which we now 

 consider under the heading " protection." From the physiological point of view the 

 consideration of the exposed situation in which the delicate, free-swimming Radiolarian 

 organism lives, and the numerous dangers which beset it in the struggle for existence, 

 would lead a 2^nori to the expectation, that many special protective adaptations would be 

 developed by natural selection. On the other hand, morphological experience shows us 

 that this latter has been in action for immeasurable periods, and has gradually produced 

 an abundance of the most remarkable protective modifications. Examples of these may 

 be found in the formation of the voluminous calymma, as a gelatinous protective 

 covering for the central capsule, and further, the formation of the capsule membrane 

 itself, which separates the generative contents of the central capsule from the nutritive 

 exoplasm. The phosphorescence of the central capsule, too (§ 223), may be regarded 

 as a useful protective arrangement ; as also the radiating of the numerous pseudopodia in 

 all directions from the surface of the calymma ; for they are of great significance to the 



