Polar capsules are structures found in the valves of
Myxosporea
Myxosporea is a class of microscopic parasites, belonging to the Myxozoa clade within Cnidaria. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate (generally an annelid but sometimes a bryozoa ...
n parasites, which contain the
polar filament. The polar capsule is constructed of a
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
aceous and a
polysaccharide 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, 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
A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this ce ...
of
jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrell ...
. The second is that extrusion is an active process involving contractile proteins and is
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
-dependent (Uspenskaya, 1982).
References
* Uspenskaya, A.V. (1984). ''Cytology of myxosporidia''.
Nauka, Leningrad. 122pp.
Animal anatomy
Myxozoa
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