We used to think of the tunicates as a rather "primitive" form of chordate .Their specific larva that looks a lot like a small, simplified fish. They probably would be better considered as a successful lineage all their own, derived from a stock that gave rise, in time, to actual chordates, but not through the tunicates.
All tunicates are marine animals and while none is found in freshwater, a relatively large number of species may live in estuaries. There are three subgroups of tunicates possessing some common characteristics.
Larvae
As do many other invertebrate groups, they possess a larva that undergoes a dramatic metamorphosis into a juvenile. Their larvae look like small tadpoles or very simplified small fishes. Having such a notochord in the tail allows them to swim much more efficiently than they could without it. The larvae and the swimming ability are transitory. Although the gut, branchial basket, and gill slits are present, they are not functional and do not open in the larva.
Only in one class, the Larvacea, do the adults retain the tadpole shape, and even so, as adults they do not swim much.
Adults
Showing other similarities to the chordates, all tunicates possess openings in the front part of their gut that are considered to be homologous with the gill slits of simple vertebrates such as lampreys. Additionally, most adult tunicates possess a rudimentary nervous system; as with many of their structures, however, the larval nervous system is more sophisticated, consisting of a main dorsal hollow nerve cord located above the notochord with very small nerves leaving it. This is a pattern also seen in the chordates.
As their name implies, many possess a "tunic" or outer supportive/protective layer. This is comprised of a type of cellulose called tunicin. Curiously, one of the few other places where cellulose is found in animals is as fibers deposited within the skin.
Only the two siphonal openings, an incurrent and an excurrent for water passage into and out of the branchial basket, pierce the tunic. Unlike other animals with a surrounding cuticle, tunicates grow inside the tunic without molting. This is probably due to their ability to resorb and redeposit tunic materials at the mantle-tunic interface. Some species have channels in the tunic that are continuous with internal blood spaces, and allow for the secretion and redeposition of tunic material as well as for the secretion of defensive materials into the tunic.
Cilia lining the edges of the gill slits pump water through the branchial basket. In this structure, mucus is produced from a ventral groove or gutter called the endostyle. A primary component of this mucus is indistinguishable from vertebrate thyroid hormone, and the endostyle is often regarded as the thyroid gland's evolutionary precursor. The mucus flows as a sheet from the ventral groove dorsally over the branchial basket's inner surfaces. The mucus, with its adherent food, is moved by ciliary action to a food groove located on the dorsal midline of the branchial basket.
From there it is moved, in the food groove, into the mouth. A very short esophagus transfers the food-laden mucus to a bag-like stomach. Associated with the stomach is a large structure called the digestive gland. The function of the digestive gland is not fully understood, but it probably contributes to digestion and absorption of food. The short, straight intestine leaves the stomach and leads to a rectum and anus. Feces are deposited in the atrium, and flushed out with the excurrent water passing out the excurrent siphon.
Tunicates are effective suspension-feeders; even small ones can filter hundreds of liters of water per day and remove well over 95% of its bacteria.
The clade Tunicata may be divided into three subgroupings. Of these, only the traditional class Ascidiacea (following most books), commonly called sea squirts or tunicates, is likely to be found in aquaria.
The other two traditional classes, the Larvacea (or Appendicularia) and the Thaliacea (salps), although almost unbelievably common in the world's oceans, are wholly pelagic and exceptionally difficult to maintain for any but the shortest time in aquaria.
Salps often form chains of connected individuals that move through the oceanic environment using the water currents produced by the connected individuals or zooids.
The tunicates possess an open circulatory system; one that is largely without vessels. Blood flows through large tissue spaces or blood channels in the tissues. Arteries, veins, and capillaries are usually absent. They have a heart, but it is a simple tube with walls that contract to force the blood through it. Unlike the heart in most animals, this heart is capable of reversing its beat. Generally, the heart beats about a hundred times in one direction, stops for a moment and then beats about a hundred times in the other direction.
Most tunicates appear to have channels for blood flow through the gills; these have sometimes been called blood vessels, but they have a very simple structure. Consequently, the gill region of the branchial basket probably functions as a respiratory organ, but little direct evidence supports this.
In some squirts, the blood contains odd rare-earth chemicals, commonly vanadium or niobium. These metals were once thought to assist in respiration, but are now known to be anti-fouling or anti-predator defenses . Other chemicals that appear to be primarily defensive are found in the blood. Blood serum containing these metals and other noxious chemicals either leaks through the epidermis, or is secreted by it and oozes through the tunic. Additional, exceedingly acidic, defensive chemicals are found in the tunic secretions of a few species. These secretions have pH values of about one.
Reproduction
All tunicates are hermaphroditic, and are often self-fertile. The gonads can develop just about anywhere in the animal. The gonoducts run parallel to the intestine and empty into the atrium near the anus. Gametes released from the gonoducts develop into non-feeding "tadpole" larvae.
The larvae exit the parent's atrial siphon and swim for a short period before selecting a substrate, often by making exceptionally precise choices . During this period they are often conspicuous and might appear to be prime food for planktivorous fishes. This does not appear to be the case, however, and it appears that some of the larvae are also protected by toxic chemicals. The larvae glue themselves, nose first, to the substrate using the secretions of anterior adhesive papillae. This is followed by an exceptionally rapid, drastic and complex metamorphosis. Epidermal cells covering the animal's surface contract and within a period of 10 to 15 seconds they crush the tail and its inner notochord, muscles, brain, nerve cord, and sensory structures into a mass of tissue debris. The rest of the viscera rotate about 90 degrees by differential growth, and the mouth and anus open. The dorsal ganglion develops from nervous system remnants. Little juvenile squirts start to feed a few days after settlement. Growth can be very rapid in small animals, with adult size reached in a few weeks. Others live longer and grow more slowly.
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Fossil records are rare but some are known from the Cambrian, Chinese sites.
