? Urochordate | ||||||
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Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates or sea squirts) is the Subphylum of saclike filter feeders with input and output siphons. But there are exceptions, like the predatory Megalodicopia hians, looking something like a cross between a jellyfish and a Venus Flytrap. They are members of the phylum Chordata, which also includes birds, fish, and mammals. As with other chordates, tunicates possess a Notochord during their early stages of development. No segmentation, not even in the tail. Metanephridia absent. The original coelom is degenerated to a pericardial cavity and gonads. Except for the Pharynx, heart and gonads, the rest of the organs in the body are enclosed in a membrane called an epicardium which are surrounded by a jelly like matrix known as mesenchyme. Larval stages may have the appearance of a Tadpole, whereas the adult stage has a much more barrel-like shape. They feed by filtering sea water through a Gill basket.
Most Tunicates are hermaphroditic. The eggs are kept inside their body, while sperm is released into the water where it fertilises other individuals when brought in with incoming water. The eggs stays inside the body until they hatch.
Tunicates consist of two openings in their body cavity. There consists an incurrent as well as excurrent siphon. The incurrent siphon is used for food and water to enter in and the excurrent siphon allows for water as well as waste to pass through and exit the tunicate. The main source of food that the tunicate consumes is Plankton. Plankton gets entangled in the mucus secreted from the endostyle. The tunicate's Pharynx is covered by miniature hairs called ciliated cells which allow the consumed plankton to pass down through to the Esophagus.
Some larval forms appear very much like primitive chordates or hemichordates with a Notochord (primitive Spinal cord). Some forms have a calcereous Spicule that may be preserved as a fossil. Jurassic to Present with one proposed Neoproterozoic form - Yarnemia.
Once grown, adults can develop a covering to protect themselves from enemies. Tunicate Blood is particularly interesting. It contains high concentrations of rare metal Vanadium and vanadium-associated proteins. Some Tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. It is still unknown how they do this or why.
They are usually divided into the classes Ascidiacea (Aplousobranchia, Phlebobranchia, and Stolidobranchia), Thaliacea, Appendicularia (=Larvacea) and Sorberacea. Newer evidences indicates that the Ascidiacea is an artificial group. The new classification would then look like this; 1) Stolidobranchia, 2) Phlebobranchia and Thaliacea, 3) Aplousobranchia and Appendicularia, and 4) Sorberacea would belong somewhere in Ascidiacea, or be in a taxon on its own. For the moment, the traditional classification are followed.