In this article we will discuss about the affinities of urochordata.
Affinities # Urochordata with Non-Chordates:
1. Food collection similar to spongec, mussels and lophophores.
2. Typhlosole in the intestine.
3. Presence of larval eye and oocytes.
4. Budding leading to a chain of new zooids.
5. Formation of colony.
1. Fish-like tadpole larva.
2. Absence of pharyngeal gill slits.
Affinities # Urochordata with Hemichordata:
1. Structure of pharynx.
2. Development of the central part of the nervous system.
1. Absence of endostyle helping in food collection in Hemichordata.
Affinities # Urochordata with Cephalochordate:
Tadpole larva and Branchiostoma: (Adult and larval structures):
1. Body shape.
2. Presence of dorsal and ventral fins.
3. Presence of velum and velar tentacles.
4. Filter feeders.
5. Cleavage and gastrulation.
6. Mode of development of notochord and nervous system.
7. Presence of notochord.
8. Similar atrial complex with endostyle, pharyngeal groove and epipharyngeal bands.
Affinities # Urochordata with Chordates:
1. Tadpole larva possesses major basic features of chordates.
2. Presence of gill slits.
3. Presence of notochord.
4. Dorsal, tubular, hollow nerve cord.
Affinities with vertebrates:
1. Tadpole larva similar to fish larva.
2. Feeding mechanism similar to Ammocoete larva and some larval amphibians.
3. Nasal glands similar to hypophysis.
4. Gonadotrophin stimulates ascidian neural gland.
Scientists are unanimous that Urochordata occupies a position in the lower rung of the hierarchy of Chordata, but they are divided in ascertaining the exact pathway of their origin.
Some suggest that the development of sexual maturity along with other complexities in the larval form led to the elimination of ancestral adult phases and the degenerated forms are the descendants of ancestors having all chordate features.
Others hold that they were lowly organized forms, which never reached the level of the simplest form of chordates. A third group believes that the appendicular larva is a primitive one giving rise to a stationary adult having degenerated vertebrate characters.
Another view is that the doliolids (urochordates) have undergone neoteny to produce present day urochordates. Regarding the interrelationship within the urochordates, it is believed that larvacea is most primitive. Thaliacea developed from fixed forms close to ancestral compound ascidians, which gave rise to Pyrosomida.