Right: Photograph of the tunicate (sea squirt) Corella parallelogramma (Müller) by Christine Ryan, note
the almost transparent outer skin or tunic, through which relatively complex
internal structures are clearly visible.
Tunicates or sea squirts possess a tadpole-like larval stage, and
while most signs of a spinal chord have been lost in the adult animal,
their internal organisation is far more complex than the sponges they
may resemble superficially.
The are unusual in having an outer skin
('test' or 'tunic') composed of carbohydrates - structural polymers more
usually associated with plants. The tunic may shelter a single animal (as here),
or a colony composed of clones budded from a parent individual.