B. The Astropodia, or free radiating pseudopodia, are in many Radiolaria more or less clearly distinguishable from the collopodia, which form the sarcoplegma within the calymma; how far these distinctions depend upon a permanent differentiation (especially in the Acantharia and Phæodaria) needs further investigation.
C. The sarcode-flagellum (perhaps better termed axoflagellum) was first described in my Monograph (1862, p. 115) in the case of various Discoidea (Taf. xxviii. figs. 5, 8; Taf. xxx. fig. 1). Hertwig has given a substantially similar account of the organ in some other Discoidea (L. N. [33], p. 67, Taf. vi. figs. 10, 11); probably this peculiar structure is confined to the order Discoidea among the Spumellaria, but is widely distributed within its limits. The axoflagellum is a thick cylindrical thread of sarcode, finely striated and pointed towards its free end. It always lies in the equatorial plane of the discoidal body, and always unpaired in one of its axes; in the triradiate Discoidea it is in the axis of the unpaired principal arm and opposite to it (Pl. [43], fig. 15). In the Ommatodiscida (p. [500], Pl. [48], figs. 8, 19, 20) the axoflagellum probably passes out through the peculiar marginal ostium of the shell. Perhaps it is always connected with the central nucleus by intracapsular axial fibres, and is to be regarded as a specially differentiated bundle of pseudopodia (or axopodia?).
95A. The Myxopodia and Axopodia.—The two forms of pseudopodia which we distinguish as myxopodia and axopodia differ markedly from each other both morphologically and physiologically. The myxopodia, or ordinary free pseudopodia, which are found in large numbers in all Radiolaria, and constitute their most important peripheral organs, are simple homogeneous exoplasmic threads, which arise from the sarcodictyum or extracalymmar sarcode network, and radiate freely into the water; here they may branch and combine by anastomosis to form a changeable network, but they never contain an axial thread. The axopodia, on the other hand, are differentiated pseudopodia, which consist of a firm radial thread, and a soft covering of exoplasm; they penetrate the whole calymma in a radial direction and project freely from its surface, and generally (if not always) they are produced inwards to the middle of the central capsule, perforating its membrane; their proximal end is lost in a dark central heap of granules. Such axopodia are at present known with certainty only in the Acantharia, where they are widely, and perhaps universally, distributed. Their development in this legion probably stands in direct causal relation to the peculiar structure of the central capsule and the centrogenous formation of the skeleton. Since the radial skeletal rods of the Acanthometra possess originally a thin coating of protoplasm, it may be said that the centrogenous axopodia of this group became differentiated in two ways, the firm axial threads of one section remaining very thin and covered by protoplasm, whilst those of the other section became metamorphosed into radial bars of acanthin. This hypothesis acquires more probability from the regular distribution and arrangement of the axopodia in the Acantharia; they usually stand at fixed intervals between the radial bars, singly or in groups; sometimes their number seems to be not greater than that of the bars, whilst in other cases a circlet or group of axopodia corresponds to each radial bar. Perhaps their fine axial thread consists of acanthin. At all events the axopodia are constant organs (probably sensory, like the "palpocils") and not retractile like the movable myxopodia.
The axial threads in the pseudopodia of the Acanthometra were first discovered by R. Hertwig, who accurately described their peculiar structure and arrangement (L. N. [33], pp. 16, 117).
96. The Myophriscs of the Acanthometra.—The Acanthometra are characterised by a very peculiar differentiation of the exoplasm, namely, by the formation of myophriscs or contractile threads from the sarcodictyum. In most (and perhaps in all) Acantharia of this order each radial bar is surrounded by a circlet of such contractile threads, which was first described as a "ciliary corona" (see note A, below). The number of contractile threads in each circlet usually amounts to from ten to twenty, rarely being more than thirty and less than eight; it often appears to be constant in the individual species (see note B). In the living state the myophriscs are long, thin filaments, the pointed distal end of which is inserted into the radial bar, whilst the thicker proximal end is attached to the surface of the calymma, which is elevated round the base of each rod into the form of a gelatinous cone or skeletal sheath (see note C). Probably the myophriscs lie on the outer surface of the apical portion of this gelatinous cone, and are hence to be regarded as exoplasmic threads differentiated from the sarcodictyum. Sometimes, however (as in Acanthochiasma), they fuse into a contractile membrane and form the envelope of a cone, whose interior is occupied by a gelatinous papilla of the calymma. On mechanical irritation the myophriscs contract rapidly and suddenly, like muscle-fibrillæ, becoming at the same time thicker, and hence are very different from pseudopodia. Their distal point of insertion being fixed to the firm acanthin rod, they raise by their contraction the skeletal sheath, to which their bases are attached or in the surface of which they lie. The result of their contraction is therefore a distention and increase in volume of the calymma, with which is no doubt connected an inception of water into the gelatinous mass, and hence a diminution in its specific gravity. Probably the Acanthometra contract their myophriscs voluntarily when they wish to rise in the water; when these relax the calymma collapses owing to its elasticity, water is then expelled and the specific gravity increases. From a physiological point of view, then, the myophriscs are to be regarded as a hydrostatic apparatus, morphologically as myophanes or muscular fibrillæ, such as also occur in the intracapsular protoplasm (see §§ [77]-[80]). On more violent irritation and after the death of the Acanthometra the myophriscs separate from the radial bars and remain attached to the distal ends of the conical gelatinous sheaths as free "ciliary coronas." At the same time, they melt into short, thick, hyaline rods, the so-called "gelatinous cilia." The myophriscs are found only in the order Acanthometra, and are wanting in the Acanthophracta, as well as in the other three legions of Radiolaria.
A. The "ciliary coronas" on the skeletal rods of dead Acanthometra were first described by the discoverer of this order, Johannes Müller, and referred to as "the stumps of the contracted, thickened threads" (L. N. [12], p. 11, Taf. xi.).
B. The "number of the gelatinous cilia" I found constant in certain species of Acanthometra, and stated in my Monograph (L. N. [16], p. 115) "that here is to be found the first differentiation of the diffuse sarcode into definite organs of regular definite number, size, and position, which deserve the name tentacles rather than pseudopodia."
C. The nature of the myophriscs as fibrillæ allied to muscles was first discovered by R. Hertwig, who described them as "structures of peculiar nature," under the name of "contractile threads," and pointed out in detail their histological and physiological peculiarities (L. N. [33], pp. 16-19, Taf. i.).
97. The Exoplasm of the Peripylea.—The extracapsular protoplasm of the Spumellaria or Peripylea is in communication with the intracapsular sarcode by the innumerable fine pores of the capsule-membrane, and like these pores is evenly distributed over the whole surface. The sarcomatrix which immediately surrounds the central capsule is moderately strong, and sends out innumerable long, thin pseudopodia, which probably correspond to the pores of the membrane. Their number is markedly greater in the Spumellaria than in the other three legions. The ramifications and communications which the radiating fibres of the sarcomatrix undergo within the calymma, apparently present the most manifold variations, so that the sarcoplegma or intracalymmar network thus formed has very diverse forms. On the surface of the calymma the exoplasmic threads constitute a variously disposed sarcodictyum, a regular or irregular exoplasmic network, by the silicification of which a primary lattice-shell arises in the majority of the Spumellaria. The free ends of the pseudopodia, which arise from this extracalymmar network and radiate out into the water, appear in most Spumellaria to be relatively short, but exceedingly numerous. Specially modified pseudopodia and axial threads in particular do not seem to occur in this legion. Perhaps, however, among the latter may be reckoned the remarkable pseudopodia which combine to form the sarcode flagellum in many Discoidea (and perhaps in other Spumellaria). This axoflagellum is a particularly strong thread of sarcode, arising from a definite point in the central capsule; it is cylindrical or slenderly conical in form, much longer, stronger, and more contractile than the ordinary pseudopodia; it contracts in a serpentine fashion on mechanical irritation and seems to originate by the fusion of a bundle of pseudopodia (compare § [95], C).
98. The Exoplasm of the Actipylea.—The extracapsular protoplasm of the Acantharia or Actipylea differs in several important respects from that of other Radiolaria, and appears to undergo more significant differentiations than that of the three other legions. Since the pores in the wall of the central capsule are not distributed evenly and at equal intervals over its whole surface (as in the Peripylea), but rather exhibit a regular disposition in groups at unequal intervals, the number of projecting pseudopodia is much less and the law of their arrangement different from that which obtains in the Peripylea (§ [58]). In many and probably in all Acantharia they are divided into two groups, those which arise from the centre of the capsule and possess firm axial threads, and those which have not these characters (compare § [95, A]). The axopodia, or stiff pseudopodia with axial threads, arise from the centre of the capsule, are present in much smaller numbers than the soft and flexible myxopodia, and are regularly disposed between the radial bars of acanthin, usually so that they are as far removed from them as possible, i.e., in the centre between each three or four bars; these latter may indeed be regarded as strongly developed axial threads, which have become changed into acanthin (§ [95, A]). The soft myxopodia, or pseudopodia without axial threads, are much more numerous than the others, and arise from the sarcodictyum or exoplasmic network which ramifies over the surface of the calymma. Their number and arrangement seem, however, in many (if not in all) Acantharia to be regular and not to possess the extraordinary variability seen in the other three legions. In many Acanthometra the sarcodictyum exhibits a symmetrical conformation, with regular or subregular, polygonal (mostly hexagonal) meshes, and generally the stronger threads of the sarcodictyum secrete a firm, homogeneous or fibrillar, striated substance, which forms a network of ridges on the surface of the calymma. In the Acanthophracta the place of this is taken by the acanthin network of the primary lattice-shell. The axopodia of the Acanthometra are usually about as long as the radial spines between which they stand; their stiff axial thread is surrounded by a soft sheath of protoplasm, communicating with the thin sarcomatrix which surrounds the central capsule. Numerous branches pass into the calymma from the exoplasmic sheath of the axial threads, and form by their interweaving a loose sarcoplegma. The most peculiar differentiated products of the exoplasm of the Acantharia, however, are the myophane fibrillæ of the Acanthometra, which have already been described under the name of myophriscs (§ [96]).