Any sessile tunicate of the class Ascidiacea.
Big saltwater raisins! Not always raisins -- some look like gooseberries, grapes, or potatoes. Some look downright inedible. They tend to be medium small (think tangerine-sized), and squirt sea water at you when you pick them up.
They have evolved past the notochord, but are humble none the less. The free-swimming larvae (looking somewhat like tadpoles) still have a notochord, but they lose it as adults. This means that they are more closely related to sea stars, lizards, and humans than to worms and raisins. They just did not find the benefits of a backbone worth pursuing.
The adults are hermaphroditic, releasing sperm into the water but holding to their eggs until they are fertilized by passing sperm and hatch. The sperm and eggs join to form a mobile larvae, which swims freely but cannot eat until it finds a place to attach itself and morph into a adult squirt.
Sea squirts are filter feeders, filtering plankton out of the water to eat. They are in turn eaten by Sharks, Skates, and other bottom dwelling fish.