The Acantharea are a small group of radiolarianprotozoa, distinguished mainly by their skeletons. These are composed of strontium sulfate crystals, which do not fossilize, and take the form of either ten diametric or twenty radial spines. The central capsule is made up of microfibrils arranged into twenty plates, each with a hole through which one spine projects, and there is also a microfibrillar cortex linked to the spines by myonemes. These assist in flotation, together with the vacuoles in the ectoplasm, which often contain zooxanthellae.
The arrangement of the spines is very precise, and is described by what is called the Müllerian law. This is easiest to describe in terms of lines of latitude and longitude - the spines lie on the intersections between five of the former, symmetric about an equator, and eight of the latter, spaced uniformly. Each line of longitude carries either two tropical spines or one equatorial and two polar spines, in alternation. The way that the spines are joined together at the center of the cell varies and is one of the primary characteristics by which acanthareans are classified.
Holacanthida - diametric spines, simply crossed
Symphyacanthida - radial spines, with free bases
Chaunacanthida - radial spines, with articulated bases
Arthracanthida - radial spines, with pyramidal bases packed together
Reproduction takes place by formation of spores, which may be flagellate, which develop into mononucleate amoebae. Adults are usually multinucleate. The axopods of Acantharea are fixed in number, and the mitochondria have tubular cristae.
All radiolarians secrete strontium sulphate at some point in the life cycle – as the adult shell in Acantharea, and as crystals in ‘swarmer cells’ produced during asexual reproduction in Polycystinea.
Phaeodarea were traditionally included in Radiolaria, and share with Acantharea and Polycystinea the traits of a glassy shell (formed of a combination of silica and organic material in Phaeodarea) and a capsule dividing the cytoplasm into inner and outer compartments.
In the Radiolaria as here defined, however, the capsule is thin and perforated by numerous pores – in Phaeodarea, the capsule is much thicker, and usually only three pores pass through it, the astropylum and and usually two parapyla situated at the opposite pole.
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