209. _Currents in the Exoplasm._--Extracapsular circulation, or a distinct flowing of the plasma outside the central capsule, may be readily observed in all Radiolaria which are examined alive; this is most readily seen in the astropodia, or those free pseudopodia which radiate from the sarcodictyum on the surface of the calymma into the surrounding water. The granular movement is often quite as clear in the sarcodictyum itself, and may be recognised in the collopodia, which compose the irregular plasmatic network within the calymma. More rarely it is possible to follow the granular stream thence through the sarcomatrix, and further into the interior of the central capsule. In general the direction of the extracapsular protoplasmic streams is radial, and it is frequently possible, even in a single free astropodium, to observe two streams opposite in direction, the granules on one side of the radial sarcode thread moving centripetally, those on the other side centrifugally. If the threads branch, and neighbouring ones {cxxxiv}become united by connecting threads, the circulation of the granules may proceed quite irregularly in the network thus formed. The rapidity and character of the extracapsular currents are subject to great variations. The different forms of extracapsular sarcode currents have been already very fully described in my Monograph (L. N. 16, pp. 89-126), and in my critical essay on the sarcode body of the Rhizopoda (L. N. 19, p. 357, Taf. XXVI.). 210. _Secretion._--Under the name _secretions_, in the strict sense, all the skeletal formations of the Radiolaria may be included. They may be divided according to their chemical composition into three different groups: pure silica in the SPUMELLARIA and NASSELLARIA, a silicate of carbon in the PHÆODARIA, and acanthin in the ACANTHARIA (compare § 102). It may indeed be assumed that these skeletons arise directly by a chemical metamorphosis (silicification, acanthinosis, &c.) of the pseudopodia and protoplasmic network; and this view seems especially justified in the case of the Astroid skeleton of the ACANTHARIA (§ 114), the Spongoid skeleton of the SPUMELLARIA (§ 126), the Plectoid skeleton of the NASSELLARIA (§ 125), the Cannoid skeleton of the PHÆODARIA (§ 127), and several other types. On closer investigation, however, it appears yet more probable that the skeleton does not arise by direct chemical metamorphosis of the protoplasm, but by secretion from it; for when the dissolved skeletal material (silica, acanthin) passes from the fluid into the solid state, it does not appear as imbedded in the plasma, but as deposited from it. However, it must be borne in mind that a hard line of demarcation can scarcely, if at all, be drawn between these two processes. In the ACANTHARIA the intracapsular sarcode is the original organ of secretion of the skeleton; in the other three legions, on the other hand, the extracapsulum performs this function (§§ 106, 107).