This phylum is mentioned in a discussion of animals without backbones only because of the existence of what appears to be a unique “missing link” between the invertebrates and the vertebrates. This unassuming animal, known commonly as the sea squirt, possesses a stiffened rod known as the notochord. This unique structure is thought to be the precursor of the vertebrate backbone. Both the notochord and its associated dorsal nerve cord appear only in larval sea squirts and are, for reasons as yet unknown, lost during the transformation to adulthood.

Sea squirts are marine animals that usually arrive in the aquarium accidentally, attached to coral, rocks, or other such animals or materials. The approximately 1,000 species are all sessile filter feeders and most are relatively small, although several types may grow to 20 inches in height.