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Hydroidolina is a subclass of Hydrozoa and makes up 90% of the class. Controversy surrounds who the sister groups of Hydroidolina are, but research has shown that three orders remain consistent as direct relatives: Siphonophorae, Anthoathecata, and Leptothecata.
Hydroidolina are small predatory animals, ranging in 8-30 millimeters in size, exhibiting radial symmetry and are Diploblasty (developed from two embryonic layers: ectoderm and endoderm).
The classification below is based on the World Register of Marine Species:
Subclass Hydroidolina
Because Hydroidomedusian polyps often settle on other organisms, they are also subject to partake in symbiotic relationships. For example, the bivalve mantle cavity of a Mollusca provides a sheltered environment, transporting food due to the current. In exchange, the hydroid protects against intruders.
are known to occur in two body forms: the polyp form which is benthic and “stalk-like,” and the medusae form, which is commonly known as the “bell” form.
Polyp forms are sessile as adults, with a single opening (the mouth/anus) to the digestive cavity facing up with tentacles surrounding it. Medusa forms are motile, with the mouth and tentacles hanging down from an umbrella-shaped bell.
Though some outlier go through a polyploid (polyp) and medusa stage, Hydroidolina, which comprises almost all hydrozoans, goes through an asexual polypoid stage where the polyp fixed to a substrate and a sexual hydroid stage varying from free-swimming medusa to a gonophore that remains attached to the polyp.
An important characteristic of the Hydroidolina is the presence and formation of an exoskeleton. The exoskeleton varies in chemical composition, structural rigidity, thickness, and coverage within the different regions of the colony and protects the coenosarc of the polypoid stage. It originates as epidermal secretions, with the exosarc being produced first by glandular epidermal cells. The exoskeleton can either be bilayered and contain both the exosarc (outer layer) and perisarc (inner layer) or corneous (just perisarc). The exoskeleton contains anchoring structures such as desmocytes and "perisarc extensions."
Within its benthic phase, polyps of these hydroids attach to soft tissues on organisms, such as the mantle of a mollusk, and reproduce asexually by budding
In the sexual medusa stage, gonophores, which are the reproductive organ that produces gametes, and will stay attached to the polyp as a reduced medusa stage but will sometimes, often rarely, form to become their own medusae.
The obsolete name Hydroida was used for a paraphyletic grouping that is now considered synonymous with Hydroidolina but did not include the colonial jellies of the order Siphonophorae.
These sessile invertebrates could prove to be useful as a measure of environmental changes within their own colonies as well as for changes within near marine environments pertaining to temporal and spatial changes to species distribution and composition, temperature, and food.
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