The Acantharea are a small group of radiolarian protozoa. Like other radiolarians, it comprises large marine cells with axopods and cytoplasm differentiated into a clear inner layer (endoplasm) and a vacuolated outer layer (ectoplasm), separated by a capsular membrane. Here the axopods are fixed in number and supported by microtubules arising from unspecified sites in the cell, and the mitochondria have tubular cristae. Reproduction is by formation of spores, which may be flagellate, that develop into mononucleate amoebae. Adults usually have many nuclei.
The Acantharea are distinguished mainly by their skeletons. These are composed of strontium sulphate 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