Acantharia
Morphology

—Characters of the skeleton
The acantharian skeleton, composed of strontium sulfate, consists of 10 diametral (or 20 radial), solid spicules which cross the cytoplasm. The distal ends of the spicules arise at precise points on the cell surface, which Müller (1859) described in an analogy to the surface of the earth. Two quartets of polar spicules emerge at 60°N and 60°S, 2 quartets of tropical spicules at 30°N and 30°S, and 1 quartet of equatorial spicules at 0°. Tropical spicules are turned 45° with respect to all others, so that the polar and equatorial tips arise at longitudes 0°, 90°, 180° and 270°, while the tropical tips arise at 45°, 135°, 225° and 315° (side view). This distribution of spicules is a general rule in the group, and although anomalous patterns have been recorded, they invariably seem to represent malformations, immature individuals, or methodological artifacts. The "acantharians" with 18 to 500 spicules described by Popofsky (1904) are now excluded from the group and considered Radiolaria incertae sedis. The "Litholophus" stage described by some authors is probably a methodological artifact (Reshetnjak, 1981).

Variability in acantharian shapes arises chiefly from differences in the development of these spicules. In over half of the species spicules are of equal length. In several families, however, 2 or 4 equatorial spicules are larger and/or different in shape from the others, and either similar or different among themselves in size and shape. Differences can also be found between polar and tropical spicules, and can in turn lead to marked changes in the overall body shape: spherical, ovoid, lenticular, etc.

The shape of the spicules is highly variable, from simple cylindrical rods, to elaborate 4- or 6-bladed structures with smooth or serrated (usually at the distal end) edges. Cylindrical spicules are probably the most primitive, and they also characterize the juvenile stages of species with more elaborate adult spicules.

Amphilithidae
Astrolithidae
Conaconidae
Phyllostauridae
Stauracanthidae

In some species (e.g., Holacanthida, Acanthochiasma fusiforme) the spicules cross loosely in the center of the cell. In others, they fuse in the center into a sphere or a star-like mass which may or may not be dissociated in SO4H2. In some forms (e.g., Acanthospira amphicopa) the spicules are flattened in the center and twisted about each other (Acanthospira amphicopa 2). The mode of junction of the spicules in the center of the shell is a most important diagnostic character.

In over half of acantharian species the skeleton is represented by the 10 spicules only, yet others develop a complementary meshwork and/or plates forming a shell. These additional structures are derived from anastomosing and merger of lateral apophyses of the spicules. Thus, each spicule can generate a pentagonal or hexagonal plate by lateral growth. This plate usually has 2 large pores right next to the spicule - the aspinal pores (pore nomenclature a). The edges of the plate can come in contact with neighboring plates, in which case the suture (or sutural furrow) can remain visible or disappear altogether. Sutural furrows are often interrupted by the sutural pores (pore nomenclature a). Alternatively, plate growth can stop short of closing the sphere (Phatnacantha icosaspis 3, Phatnacantha icosaspis 5). Smaller coronal pores are scattered between the aspinal and the sutural ones (pore nomenclature d).

Acantharian shells may be either a delicate, densely perforated meshwork (Dictyacantha tabulata), or robust and thick-walled (Coleaspis coronata 2). The basic shell pattern may be modified further in various groups by additional structures. Thus, in Phractopeltidae there are 2 (rather than 1) concentric spheres (Phractopelta tessaraspis 2). In Lithopteridae large wing-like meshwork apophyses develop on the distal ends of the 2 main spicules (Lithoptera fenestrata 2, Lithoptera muelleri 2). In Diploconidae the ellipsoidal shell is provided with two large opposite conical "sleeves" (cornets) or mantle (Diploconus cyathiscus, Diploconus fasces 2,Diploconus saturnus 3).

Although dimensions of the skeleton's elements are an important diagnostic feature, size is subject to wide geographic variability. Nencini and Saglina (1968), for example, showed that the main spicules of Atlantic specimens of Amphibelone anomala (Amphibelone anomala) are over twice as large (380 µm) as those collected in the Mediterranean (170 µm). The overall size, aspect, and complexity of the skeleton also changes markedly with growth Successive stages of growth.

—Cell body organization
The cell body consists of a central endoplasm and a peripheral ectoplasm. In most species the endoplasm, which contains the nuclei, is colored brown, red, or black by pigments. In 3 orders (Holacanthida, Symphyacanthida, Chaunacanthida) the endoplasm is dense in the cell center and becomes progressively clearer towards the periphery. Moreover, it is not distinctly separated from the ectoplasm. In contrast, in the fourth order (Arthracanthida), the endoplasm is easily distinguished from the ectoplasm since it is limited by a thick capsular wall. The endoplasm includes nuclei and most cell organelles such as large mitochondria, ribosomes, areas of rough endoplasmic reticulum, dictyosomes and various membrane-bound organelles. In some Symphyacanthida (Haliommatidium and Dicranophora), during the vegetative life the endoplasm contains a single large nucleus, probably polyploid, while in all other Acantharia there are numerous small round or oblong nuclei. In many species, the trophont contains hundreds to thousands of nuclei immediately prior to gametogenesis.

In the Holacanthida, Chaunacanthida and Symphyacanthida, which encyst just before gametogenesis, the endoplasm is progressively enriched in tiny oval plates - the lithosomes. They are generated by the Golgi and are birefringent. They are transported towards the cell surface and arranged to form the cyst wall (acantharian cysts 1). During this morphogenetic change the external networks and the myonemes are shed.

Axopodia, slender processes radiating from the cell surface, are sensitive and can retract upon chemical or physical stimulation, then re-grow progressively and slowly. They are stiffened by a central axis (axoneme) consisting of patterned arrays of microtubules.

The cytoplasm around each spicule is limited by the cell membrane and contains 2-60 myonemes (depending on the species). Myonemes are birefringent fibrillar bundles anchored by one end to the spicules and by the other end to the periplasmic cortex (schematic cross section, Amphibelone hydrotomica 2). Their lengths are in the range of about 5-90 µm. They are either flat, triangular, or as long ribbons or short cylinders. The myonemes are motor organelles capable of active movement. Rapid and simultaneous contraction of all the myonemes produces inflation of the cortex and the volume of the ectoplasmic region is consequently increased. Such a coordinated activity of the myonemes may play a part in buoyancy regulation (Febvre-Chevalier and Febvre, 1994).