Latin: chorda = cord
The phylum Chordata has
three Subphyla, the Urochordata, the Cephalochordata and the the Vertebrata. Only the first two are invertebrates.
The chordata have an endoskeleton so they can grow continuously without moulting. The endoskeleton provides a framework for muscles to attach to. According to recent molecular research it is believed that the Chordata evolved from the Echinodermata.
These animals have a notochord and nerve cord in the free-swimming larval stage only. The adults are sessile and encased in a tunic. They have no excretory organs and are hermaphrodite. They are found in all seas at all depths. The larval form is tadpole-shaped.
There are 70 species and all
are planktonic. They have barrel-shaped, transparent, gelatinous bodies and some live colonially.
Many have luminous organs Asexual and sexual reproduction is possible and also budding.
There are 70 known species,
all marine and about 5 mm in length. They have a long tail throughout their
life, and inhabit a walnut-sized gelatinous test. The beating of the tail
causes water to be drawn in and filtered particles are eaten. The filters in the gelatinous test become clogged with waste, and the animal abandons it and builds a new one, which takes only minutes. They are never
colonial and reproduction is always sexual.
These are the sea
squirts, and are the most common class, there are about 2000 species, all marine. There are solitary, colonial and compound species. Compound 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 below 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.
Sea squirts often cause problems by colonising buoys, the hulls of ships and piers.