Acantharia

“ Adelacantha and Icosacantha.—The numerous forms of Acantharia, here disposed in twelve families and sixty-five genera, may be divided phylogenetically into two main groups of very different extent—Adelacantha and Icosacantha. The more primitive group, Adelacantha, have an indefinite and variable number of radial spines, which are always quite simple in form and usually irregularly distributed; this main division includes only the one order Actinelida, with six genera, among which is Actinelius, the common stem-form of all the Acantharia. ”

“ Nevertheless the small group of Actinelida is very important, being probably the ancestral group from which all other Acantharia have been phylogenetically derived. These other Acantharia, with twenty regularly disposed radial spines, represent the two large groups of Acanthonida and Acanthophracta. For short and clear distinction of these two groups of Acantharia, we will call the Actinelida (with irregular number and disposition of radial spines) Adelacantha, in opposition to the Icosacantha (Acanthonida and Acanthophracta) , which all possess twenty regularly disposed radial spines. ”

“ Definition.—Acantharia with twenty radial spines of very unequal size, disposed according to the law of the Icosacantha; two opposite equatorial spines (in the longitudinal axis) much larger than the eighteen others. No lattice-shell.
The family Amphilonchida is distinguished from all other Acanthometra by the excessive development of two opposite equatorial spines, which are much larger {782}than the eighteen other spines. In my Monograph (1862, p. 389) I had united all these Acanthonida in a single genus Amphilonche (with ten species) . ”

“ The Nucleus of the Actipylea.—The nucleus of the Acantharia or Actipylea shows very peculiar relations in respect of structure and division, particularly special forms of lobular budding, which belong to the characteristic peculiarities of this singular legion, and are not found among other Radiolaria. The position of the nucleus is always excentric, even in the youngest Acantharia, for the centrogeneous formation of the skeleton, the constant development of the earliest radial portions of it in the middle of the central capsule, forces the nucleus from its normal central position. ”

“ Both the organic acanthin skeletons of the Acantharia and the silicate skeletons of the Phæodaria, as well as the inorganic siliceous skeletons of the Spumellaria and Nassellaria, appear under the microscope perfectly homogeneous, transparent, colourless, and crystalline. Only very rarely do they show traces of a concentric striation, which arises from the deposition of the skeletal substance in layers; as, for example, the thick spines of some Phæodaria (Pls. 105-107, &c.) ”

“ The refractive index of acanthin is, however, very different from that of glycerine, so that the skeletons of Acantharia are readily visible when mounted in this fluid. In water, the skeletons of all Radiolaria appear about equally refractive, as also in Canada balsam. The substance of the skeleton appears almost entirely hyaline, colourless, and transparent. Very rarely it is faintly coloured (in some Acantharia) . A cloudy opaque constitution is seen in some Phæodaria (especially in the "porcellanous shells" of Tuscarorida and Circoporida, Pls. 100, 114-117) ”

“ To this category belong all Phæodaria and Nassellaria, as well as all the solitary or monozoic Spumellaria and some Acantharia. On the other hand, the name "precocious," or early dividing, is applied to those Radiolaria in which the division of the nucleus takes place very early, and in which, therefore, the cell is multinuclear during the greater part of its existence. This is the case in all the social or polyzootic Radiolaria (Polycyttaria, Pls. 3-8) , and also in the great majority of the Acantharia, both Acanthometra and Acanthophracta. ”

“ The more recent group, Icosacantha, includes all the other Acantharia (fifty-nine genera) , and is very markedly distinguished from the Adelacantha by the fact that the radial spines are always twenty in number, and arranged according to Müller's law (compare pp. 717-725, and § 110) . Since this regular disposition (in five alternating zones each of four spines) has been retained by inheritance in the whole of the Icosacantha, it is probable that this large group has been developed monophyletically from a twig of the Adelacantha ”

“ The spherical ground-forms, fully developed in the central capsule and calymma of Actissa and the Sphæroidea as well as in many Acantharia, present no different axes; all possible axes passing through the centre of the body are equal (Homaxonia) . ”

“ The numerous genera of Icosacantha may then be again divided into two suborders, of which the Acanthonida (p. 740, Pls. 130-132) produce no complete lattice-shell, and thus agree with the Actinelida, with which they may be united as Acanthometra in the broader sense (or Acantharia without a lattice-shell) . The Acanthophracta, on the other hand (p. 791, Pls. 133-140) , produce a complete lattice-shell, usually by means of two opposite or four crossed transverse processes, which arise from each radial spine and unite with each other (see note B, below) . ”

“ The Central Capsule is constantly spherical in the far greater number of the Acantharia, viz., in the following six families:—Astrolophida, Chiastolida, Astrolonchida, Dorataspida, Sphærocapsida, and Phractopeltida. Among these six families the Astrolonchida and Dorataspida are far greater and far richer in different forms than all the other families. The central capsule becomes ellipsoidal or cylindrical, prolonged in one axis, in the three families, Amphilonchida, Belonapsida, and Diploconida ”

“ Of the four legions of the Radiolaria, the Acantharia (on account of the solubility of their astroid acanthin skeletons) have entirely vanished and have never been found fossil. Of the Phæodaria, whose silicate skeleton is not as a rule capable of fossilisation, only one section (Dictyochida) of a single family (Cannorrhaphida) has been observed fossil. Hence the fossil remains of the Radiolaria belong almost exclusively to the two legions, Spumellaria and Nassellaria, which were formerly united under the term "Polycystina." ”

“ They do not become yellow either with nitric acid or with iodine. In dilute mineral acids they dissolve more rapidly than in concentrated. My usual method of cleansing the skeleton of Acantharia (which has been practised with the same result on thousands of specimens) consists in heating the preparation in a small volume of concentrated sulphuric acid and then adding a drop of fuming nitric acid; all other constituents (the whole central capsule and the calymma) are thus very rapidly destroyed ”

“ Their elasticity also is very small, whilst that of the acanthin spines is considerable. The thin long needles of many Acantharia are very elastic, as are also the bristle-like siliceous spicules of many Spumellaria. The refractive power of the skeleton in the various legions is very different, depending upon the chemical constitution. The siliceous skeleton of the Polycystina (Spumellaria and Nassellaria) and the silicate skeleton of the Phæodaria have the same refractive index as glycerine, and hence become invisible when mounted in that fluid ”

“ The Apophyses of the Radial Spines, or their "lateral transverse processes," are of the greatest importance for the morphological development of the whole subclass. Only in sixteen among the sixty-five genera of Acantharia are the apophyses perfectly wanting; in the other genera they determine in the first place their general character. In the Acanthometra the apophyses remain perfectly free, whilst in the Acanthophracta their meeting ends or branches compose the latticed shell. All differences in form and shape of the apophyses can be reduced to only two primary modes ”

“ Such indifferently stable bodies are found among the Spumellaria in many Collodaria and Sphæroidea, as well as in the Astrolophida among the Acantharia. On the contrary they are entirely wanting among the Nassellaria and Phæodaria, since their central capsule constantly presents a main axis with a differentiated basal pole, and determines the position of stable equilibrium. ”