Did you know that humans are related to sea squirts? Although you may not see the family resemblance, sea squirts (class Ascidians), are members of the phylum Chordata, which include all animals with a spinal cord (e.g. birds, fish, reptiles, and humans). However, unlike typical chordates, sea squirts only have their spinal cord and a primitive brain for part of their life. In their larval stage, sea squirts look similar to tadpoles; they have a spinal cord, simple eye, and a tail for propulsion. The little sea squirt swims until it finds a suitable place to spend the rest of its life, attaching itself to rocks, boats, or even the backs of crabs. Once sessile (immobile), there is no further need for locomotion, so the sea squirt reabsorbs its brain and spinal cord.
Fact and sketch by Karli Chudeau
Source: Meinertzhagen, I.A. & Okamura, Y. (2001). The larval ascidian nervous system: the chordate brain from its small beginnings. Trends in Neurosciences, 24(7) DOI: https://doi.org/10.1016/S0166-2236(00)01851-8.