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Tunicates

Tunicates (Tunicata) are, together with vertebrates and cephalochordates, the three subphyla of animals characterized by having a nervous cord that runs through the body of the animal and that make up the phylum of chordates (Chordata). Within the subphylum of tunicates we find 3 classes (Appendicularia, Ascidiacea and Thaliacea) that congregate 7 orders and a total of approximately 3,000 different species, all of them inhabitants of marine waters. The name tunicates comes from the substance secreted from their body wall, tunicine. Tunicine is a celluloid-like substance that gives the specimens a supporting structure which could be considered the skeleton of the animal. The body of tunicates consists of a sac-like structure, with a central cavity called the atrium, which is filled with water. There are two tubular openings that communicate the atrium with the exterior, which are called siphons. Under the external surface of the body wall, we find muscular bands oriented longitudinally, which allow them to retract the siphons and thus manage to pass water through their atrium. With this movement of water and filtering the organic particles present in the water, tunicates obtain their food source.

There are tunicates with different types of mobility, fixed species in the marine substrate (benthic), species that live in the water column (planktonic), as well as solitary and colonial species.

The body of tunicates is divided into thorax and abdomen, and their nervous system is very reduced. They have a small cerebroid ganglion above the pharynx, from which some nerves derive and are directed towards the muscles of the rest of the body. During the larval stage, tunicates have a well-developed dorsal nerve cord. This nerve cord is lost when the specimens reach their adult stage.

Regarding the reproduction of tunicates, they are hermaphrodite animals whose fertilization is usually external by releasing the gametes into the water. Fertilization occurs in the same water column, forming a larva that becomes part of the zooplankton. After some time, the larvae are deposited on the seabed in benthic species.



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