THE RADIOLARIA 



135 



a casual manner, and probably serving to increase the extent of 

 exposed surface. Cannosphaera from Antarctic seas .possesses a 

 hollow skeleton the tubes of which are almost filled with masses of 

 the diatom Corethron ; and finally, in Aulokleptes (Fig 19) and 

 Aulodendron the diatoms are planted radially in the ectoplasm, 

 surrounded by a mucilage, and 

 finally incorpoi'ated into the walls 

 of a hollow radial tube, the lamellae 

 of which are laid down from within 

 outwards, and the top of which is 

 moulded into the form severally 

 characteristic of the species (Immer- 

 mann, Hacker). 



The biological significance of 

 the varieties of Nassellarian spicule 

 and of the scattered Spumellarian 

 spicules and lattice - shells is at 

 present quite obscure, but the 

 skeleton of the Acantharia offers 

 perhaps the clearest case of func- 

 tional significance to be found in 

 the whole group (Dreyer, Brandt, 

 Popowsky). The twenty radial 

 spokes of the Acanthometrea serve 

 as so many tent-poles for the in- 

 sertion of the myonemes (Figs. 11 

 .and 18) that hoist the calymmal 

 cones. This action, combined with absorption of water into the 

 vacuoles, causes a swelling of the cytoplasm and brings the 

 animal towards the surface ; whereas relaxation of the myonemes 

 and contraction of the calymma depresses it beyond the reach of 

 wave-action. The skeleton of this subdivision is, however, related 

 to hydrostatic ends in another way. The definite arrangement of 

 the twenty spines according to what is known as Miiller's law 

 (p. 132) has recently been correlated with flotation and dispersal. 

 Brandt has shown that the distribution of the radii in five alternate 

 and superposed circles, each of four spicules, is such as to expose 

 them freely and without overlapping to the viscosity and resistance 

 of the water. The absence of vertical or axial spines is also intel- 

 ligible, since they would increase the weight of the body without 

 giving additional buoyancy. Moreover, the shape as well as the 

 arrangement of the spines assist the Acantharia in their flotation 

 and dispersal. Like all other Radiolaria, these are dependent on 

 currents and drift for their dissemination. In order to utilise this 

 horizontal force, the radial spokes are frequently provided with four 

 flanges or blades, which serve the double purpose of encountering 



Fio. 10. 



Spicule from Aulokleptes floscvlus formed 

 around a dividing diatom Rhizosoknia. 

 (After Immermann.) x 55. 



