Acantharia

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A subclass of Actinopodea. These marine protozoans are related to the Radiolaria and possess a nonliving, organic capsular wall surrounding a central mass of cytoplasm. The intracapsular cytoplasm is connected to the extra-capsular cytoplasm by fine cytoplasmic strands passing through pores in the capsular wall. Skeletons are typically constructed of celestite (strontium sulfate) instead of silica. The basic structural elements are 20 rods which pass through the capsule to the center in regular arrangement (polar and equatorial; see illustration). An equatorial rod forms an angle of 90° with a polar rod, and other groups are arranged with similar exactness. This type of skeleton may be modified by addition of a latticework, apparently composed of plates, each fused with a skeletal rod. Some genera show a double latticework, concentric with the central capsule.

Acantharia: Acanthometra pellucida.

Pseudopodia are more or less permanent. Zooxanthellae are of at least two kinds: dinophyceae containing trichocysts in the cytoplasm; and a group of algae characterized, among other things, by numerous discoidal plastids each of which contains an interlamellar pyrenoid.

Myonemes (myophrisks) are significant components of the hydrostatic apparatus and apparently regulate the buoyancy of Acantharia by expanding or contracting portions of the extra-capsular cytoplasmic sheath.

Although essentially pelagic, Acantharia may move vertically with the help of their hydrostatic apparatus. Little is known about the ecology or distribution of Acantharia. The Gulf Stream is rich in Acantharia in spring and summer, but rather poor in other seasons.

The taxonomy of Acantharia is under review, but according to one scheme there are two orders: Acanthometrida, and Acanthophractida. See also Acanthometrida; Acanthophractida; Actinopodea; Protozoa; Sarcodina; Sarcomastigophora.