The tunicate, also known as the "sea squirts" are best known for their vast colors and unique shapes and sizes. The tunicate will usually stay in one place most of their life, living on rocks, towards the bottom of the ocean. Tunicates sometimes form large colonies, where they all live together, very tightly. There are a vast number of varieties of tunicates including the blue bell tunicate (Clavelina puertosecensis), the light bulb tunicate (Clavelina oblonga), the translucent tunicate (Ciona intestinalis) and more. Interestingly, some parts of the tunicates can be used for medical purpose to fight cancer.
The tunicate has three types of chordate structures. These structure may or may not appear in either the larval or adult stage of their life. If you go inside of the tunicates central cavity, you will see that they have many gill slits, as apart of their respiration activities.  The tunicates are mostly sedentary, which means that they stay in one place, on one rock, most of their lifetime. Their tunic (or their hard, protective body covering) is made up of cellulose, and that is where its name originated from.  The tunicates are usually in the form of some sort of cylinder. Some have opens on both sides, others however, don't.  At the larval stage, the tunicate has a notochord, brain, and gills. It does create a mouth at the larval stage, but is never used or open at anytime during this stage.  At the adult stage, the tunicate has many more structures. Along with the brain, notochord, and gills, it also develops a digestive system and a circulatory system. There is a sac by one of the siphons that takes the oxygen from the water, so the tunicate can breath. Food also comes in through one of theses siphons. At the other end, there is another siphon, that lets out all the waste and things that did not digest.  Their circulation is controlled by the blood vessels that circulate through it's exoskeleton. 
Tunicates can reproduce asexually and sexually. Some tunicates can reproduce by budding, which are performed differently by different types of tunicates. Even though some can perform asexual reproduction, all tunicates have the ability to reproduce sexually, and many of them are hermaphrodites. The tunicates have one testes and the ovary organ, that are inside of the body. The fertilization takes place on the outside, and the eggs are produced inside. When the larva leaves the tunicate, it then hatches. The tadpole is a small organism, with a tail that helps it move its way around in water. These tadpoles have stiffening rods, a nerve cord, and some gills.  Once the the larva hatches, it then will find a rock, and attach itself to it. This is when the tunicate changes the most, and grows into adult form; but this larval stage cannot feed on anything. When they become adults, a tunic covers there body for protection. 
How it feeds- The tunicate is a suspension feeder. What brings in the food and water is a system called the incurrent siphon. Then the excurrent siphon gets rid of the waste and leftover water that was in the body. 
What it feeds on- The tunicate mostly searches for plankton. When the tunicate catches the plankton, it is caught in the tunicate's pharynx, which then goes down through the esophagus. Then, the waste of their food is directed out of the anus, into the water. 
Types of Tunicates- there are free-swimming tunicates, sea squirts, benthic tunicates, and salps. 
You can usually find tunicates in marine environments, attached to a rock, and they can grow about one to twelve centimeters in length. Not all tunicates are the same color either. Their colors are among a vast array of the color scale, they can be dull green, all the way up to bright colors. 
Tunicates can form large colonies, or simply live by themselves. The colonies can be one meter or larger in length. Most of them are closer to the edges of oceans, than found way in the depths of the oceans. Some have been recorded though, to be seen in two-hundred meters deep of water. They aren't always shaped evenly either; they usually are shaped irregularly or spherically. 
It's hard to find a full tunicate fossil, since its body is hard to keep in the same shape. That's why there aren't really many tunicate fossils that have been recorded. Sometimes, the spicules that come off of the tunicate are found, and recorded as fossils, but that is the most likely way they can ever determine if there were tunicates far back. Since sponges also have spicules on their bodies, it is sometimes a confusion to whether or not the spicule has come from a tunicate or a sponge. The Yarnemia, has been recorded as a tunicate fossil, but it is still unknown to whether or not it is an actual tunicate or a sponge. 
Tunicates: Cancer Fighting Medical Uses
Surprisingly, some parts of the tunicates can be used for medical purposes. The tunicate has three types of chemicals/structures that can help prevent/ decrease cancer. Didemnins, Esteinascidin, and Aplidine all have effects on some different kinds of cancer. 
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