Characteristics of Urochordata:
- Possesses a Notochord, a hollow nerve cord and a post anal tail.
- Body has more than two cell layers and includes tissues and organs.
- Has a U shaped gut.
- Body has no coelomic body cavity.
- Body wholly enclosed in a ‘tunic’ of secreted protein and cellulose-like material.
- Are hermaphroditic, normally with only one ovary and testis.
- Has a nervous system composed of a anterior ganglion from which individual nerves issue.
- Has no excretory organs.
- Has a distinct larval stage.
- All are filter feeders.
- Live in marine environments.
- About 2,000 species currently known.
The Urochordata are a medium sized group of around 2,000 species of marine animals, commonly referred to as Sea Squirts, Tunicates, Salps and Larvaceans.
They are all filter feeders, using a basically similar mechanism of pumping water through a perforated (having holes in it) pharynx which collects small particles in a layer of mucous.
All the Urochordates possess an external covering or ‘house’ of secreted proteins and a polysaccharide much like cellulose. In some cases this matrix contains living cells that have migrated from the main body of the animal, and even sometimes blood vessels.
The animal lives within its house permanently in most cases. The exception being the larvaceans whose ‘houses’ are less substantial and who secrete a new house every four hours or so.
Although the Urochordates are close relatives of the chordates and thus of vertebrates such as mammals like us, they seem to be far less like vertebrates than many of the other invertebrate phyla. They have no limbs, no brain and – except in the larvaceans – the tail is only evident during larval development.
The subphylum is divided into 3 classes: