The Acantharea are a group of radiolarian protozoa

Protozoa

Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

, distinguished mainly by their skeletons.

Structure

These are composed of strontium sulfate

Strontium sulfate

Strontium sulfate is the sulfate salt of strontium. It is a white crystalline powder and occurs in nature as the mineral celestine. It is poorly soluble in water to the extent of 1 part in 8,800. It is more soluble in dilute HCl and nitric acid and appreciably soluble in alkali chloride solutions...

 crystals, which do not fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

ize, and take the form of either ten diametric or twenty radial spines. The central capsule is made up of microfibril

Microfibril

The microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, examples are hair and sperm tail. Its most frequently observed structural pattern is 9+2...

s 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 vacuole

Vacuole

A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...

s in the ectoplasm, which often contain zooxanthella

Zooxanthella

Zooxanthellae are flagellate protozoa that are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans such as the scleractinian corals and the tropical sea anemone, Aiptasia....

e.

Classification by spine arrangement

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
  Arthracanthida
  Arthracanthida, a sub-class of Acantharea, is a group of marine protozoans. They consist mainly of a gelatinous sheath filled with cytoplasm and a skeleton of up to 20 radially placed spicules made of celestite. While mostly found in the upper areas of the ocean, they are able to move vertically by...
  
   - radial spines, with pyramidal bases packed together

The axopods are fixed in number.

Life cycle

Adults are usually multinucleated. Reproduction is thought to take place by formation of swarmer cells (formerly referred to as "spores"), which may be flagellate

Flagellate

Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...

. Not all life cycle stages have been observed, and study of these organisms has been hampered mainly by an inability to maintain these organisms in culture through successive generations.