sea squirt

Fossil range: Early Cambrian - Recent

Sea Tulips, Pyura spinifera Scientific classification Kingdom: Animalia

Phylum: Chordata

Subphylum: Urochordata

Giribet et al., 2000 Classes

Ascidiacea (2,300 species)

Thaliacea

Appendicularia

Sorberacea

Tunicate, also known as urochordata, tunicata (and by the common names of urochordates, sea squirts, and sea pork[1]) is the subphylum of a group of underwater saclike filter feeders with incurrent and excurrent siphons, that are members of the phylum Chordata. Most tunicates feed by filtering sea water through pharyngeal slits, but some are sub-marine predators such as the Megalodicopia hians. Like other chordates, tunicates have a notochord during their early development, but lack myomeric segmentation throughout the body and tail as adults. Tunicates lack the kidney-like metanephridial organs, and the original coelom body-cavity develops into a pericardial cavity and gonads. Except for the pharynx, heart and gonads, the organs are enclosed in a membrane called an epicardium, which is surrounded by the jelly-like mesenchyme. Tunicates begin life in a mobile larval stage that resembles a tadpole, later developing into a barrel-like, sedentary adult form.

While most tunicates live on the ocean floor, salps, doliolids, and pyrosomes live above in the pelagic zone as adults.

Tunicates apparently evolved in the early Cambrian period, beginning c 540 million years ago. Despite their simple appearance, tunicates are closely related to vertebrates, which include fish and all land animals with bones. Contents [hide] 1 Life cycle 2 Feeding 3 Classification 4 Fossil record 5 Invasive species 6 Medical uses 7 References 8 External links //

Life cycle

Most tunicates are hermaphrodites. The eggs are kept inside their body until they hatch, while sperm is released into the water where it fertilizes other individuals when brought in with incoming water.

Some larval forms appear very much like primitive chordates or hemichordates with a notochord (primitive spinal cord). Superficially the larva resemble small tadpoles. Some forms have a calcereous spicule that may be preserved as a fossil. They have appeared from the Jurassic to the present, with one proposed Neoproterozoic form, Yarnemia.

The larval stage ends when the tunicate finds a suitable rock to affix to and cements itself in place. The larval form is not capable of feeding, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body, one of the most interesting being control of movement by the digestion of the cerebral ganglion, which is the equivalent of the human brain in some arthropods and insects. From this comes the common saying that the sea squirt "eats its own brain".[2] In some classes, the adults remain pelagic (swimming or drifting in the open sea), although their larvae undergo similar metamorphoses to a higher or lower degree.

Once grown, adults can develop a thick covering,