These will never grow up
Appendicularia or Larvaceans are small tadpole like, non-vertebrate chordates, meaning they don’t have a spine, but they have a sort of spinal cord known as a notocord.
I may have called them tadpole-like, but it would be more accurate to compare them to the juvenile form of another non-vertebrate choradate, the ascidians to which they are very similar. In fact it is believed that these bizarre little creatures evolved as a result of a process called neotony, in which the organism reaches maturity in what is typically a juvenile phase.
They are filter feeders, but due to an interesting little adaptation they are able to eat almost any organic material no matter how small. They build a little very sticky external skeleton known as a house, which usually surrounds the whole body. This bizarre structure attaches to all particulate organiganic matter floating by which is concentrated before it is eaten. The house loses its effectiveness over time, and the Appendicularian can discard it when this happens.
Appendicularians are found throughout the world’s oceans.