The chordata have an endoskeleton so they can grow continuously without moulting. The endoskeleton provides a framework for muscles to attach to. Accordin to recent molecular research it is believed that the Chordata evolved from the Echinodermata.

SUBPHYLUM UROCHORDATA

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.

Class Thaliacea

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.

Class Larvacea

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.

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.

SUBPHYLUM CEPHALOCHORDATA

These are the lancelets, there are 25 known species. They are usually less the 10 cm long and free living. They are sedentary and are usually partly buried in the sand in coastal waters, however they do swim when changing location. They are fish-like, but without a differentiated head. Their body is translucent and they have v-shaped muscle blocks. They have a hollow, dorsal nerve cord which extends almost the whole length of their body throughout their lives. Adult lancelets resemble larval tunicates (see above), and possibly evolved from them. This retention of juvenile characteristics into adulthood is called neotony. Lancelets have been in existence since the Cambrian.

Amphioxus lanceolatus, left, has no distinct head, and is found in shell gravel or coarse sand. It burrows head end first down into the sand, and then turns upwards until its head end emerges. It is a filter feeder. The water passes through the oral spines which stop larger particles and debris, then into the mouth and pharynx, and across the gills.