Sea squirts are very interesting. When you see one in its adult form, it looks like a sea sponge that can squirt water. But, when you were a larvae, you have all the characteristics of a chordate, an animal with a spinal cord.
Introduction to the Urochordata

Tunicates. Photo by Crissy Huffard, UCMP.
The Urochordata, sometimes known as the Tunicata, are commonly known as "sea squirts." The body of an adult tunicate is quite simple, being essentially a sack with two siphons through which water enters and exits. Water is filtered inside the sack-shaped body. However, many tunicates have a larva that is free-swimming and exhibits all chordate characteristics: it has a notochord, a dorsal nerve cord, pharyngeal slits, and a post-anal tail. This "tadpole larva" will swim for some time; in many tunicates, it eventually attaches to a hard substrate, it loses its tail and ability to move, and its nervous system largely disintegrates. Some tunicates are entirely pelagic; known as salps, they typically have barrel-shaped bodies and may be extremely abundant in the open ocean.

Urochordates have a sparse fossil record. A Precambrian fossil known as Yarnemia has been referred to the Urochordata, but this assignment is doubtful. Complete body fossils of tunicates are rare, but tunicates in some families generate microscopic spicules that may be preserved as microfossils. Such spicules have occasionally been described from Jurassic and later rocks. Few paleontologists are familiar with them; tunicate spicules may be mistaken for sponge spicules. Shown below is a spicule from a living tunicate from Moorea, French Polynesia, photographed using an Environmental Scanning Electron Microscope.