Polar capsules are structures found in the valves of Myxosporea

Myxosporea

The Myxosporea are a class of microscopic parasites, belonging to the Myxozoa. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate and an ectothermic vertebrate, usually a fish. Each host releases a different type of spore...

n parasites, which contain the polar filament

Polar filament

A polar filament is a structure found in the polar capsule of myxosporean organisms, and is analogous to the "penetrant" structure found in cnidocytes....

. The polar capsule is constructed of a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

aceous and a polysaccharide

Polysaccharide

Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...

 layer, both layers of which continue into the polar filament.

The mouth of the capsule is covered with a cap-like structure. This structure may function as a stopper

Stopper (plug)

A bung is truncated cylindrical or conical closure to seal a container, such as a bottle, tube or barrel. Unlike a lid which encloses a container from the outside without displacing the inner volume, a bung is partially inserted inside the container to act as a seal...

, its digestion in the alimentary tract possibly triggering the discharge of the polar filaments.

Two ideas have been proposed to explain the eversion of the polar filaments. Firstly, that the hydrostatic pressure in the polar capsule pushes the filament out, rather like the cnidocyst of coelenterates. The second is that extrusion is an active process involving contractile proteins and is calcium

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

-dependent (Uspenskaya, 1982).