larval stage of sea squirt
They are filter feeders drawing water in through the incurrent siphon (mouth) usually located at the top (see right), and expelling water and waste through the excurrent siphon. The food particles are caught in a mucous net and passed to the intestine by cilial action. They get their common name of sea squirts because they can squirt a jet of water out of the excurrent siphon if they feel threatened.
These are the sea
squirts, and are the most common class, there are about 2000 species, all marine. There are solitary, colonial and compoud species. Compoud individuals share the same tunic and may have a common excurrent siphon.Sea squirts are also called tunicates because a tunic of cellulose-like substance envelops the body of adults, see the drawing below right. The larval stage is
usually free-living and the body shape is similar to a tadpole (see left). They have a notochord during their larval stages only. This attaches by the head to the substrate, and differential growth leads to
the mouth migrating to what is now the upper end.
Ascidians have an unusual
heart which pumps blood in two directions reversing after a few beats. Many species are colonial, and asexual reproduction by budding is
common. They also have the ability to regenerate their entire body form from a fragment. So if a sea squirt is cut up in pieces, each piece containing a blood vessel can regenerate a whole new sea squirt.
above adult stage of sea squirt inside its cellulose-like tunic