Metamorphosis is a change from the juvenile to adult stage in which larval stage is quite different from
the adult stage.
In retrogressive metamorphosis the larva possesses advanced characters which are lost during the development and the adult is either sedentary or degenerated with primitive characters.
Urochordate adults, being sedentary show degenerative characters while the free swimming tadpole larva shows advanced chordate characters which are lost during metamorphosis (axial notochord, dorsal neural tube etc.).
Larva of Herdmania
Tadpole larva of Herdmania is only 1-2 mm long when it hatches out of the egg.
It does not feed and hence has only 3 hours of survival during which it has to swim in search of a suitable substratum for attachment.
The larva needs advanced features for its free swimming existence, which is also necessary for dispersal of the population to distant places.
The advanced Chordate characters of the larva.
Chordate features
Rod-like notochord in the tail to which are attached muscle bands for swimming.
Dorsal hollow nerve cord which is enlarged to form brain at the anterior end.
Photoreceptor ocellus and a balancing organ (statocyst) are attached to the brain.
Two pairs of gill slits in pharynx but the mouth is closed by a membrane and intestine is rudimentary.
Endostyle on the ventral side of pharynx is very well developed which functions like thyroid gland and helps in metamorphosis.
Heart is on the ventral side of gut but is non-functional.
Changes during metamorphosis
Retrogressive changes
Larva attaches to the substratum with the help of chin warts, head downward and tail up.
Rapid growth takes place between the chin warts (adhesive papillae) and mouth and almost no growth on the opposite side of body.
Adhesive Papillae disappear completely.
Due to rapid growth on one side, body starts rotating in such a way that mouth gradually migrates to the upper side.
Both tail and caudal fin are gradually absorbed in the body or cast off during metamorphosis.
Sense organs, namely ocellus and statocyst are lost.
The hollow nerve cord is reduced into a solid nerve ganglion on the dorsal side. Notochord disappears and is consumed by phagocytes.
Progressive Changes
Intestine becomes functional and atrial opening is formed on the opposite side of oral aperture.
Transformed into a bag-like sedentary animal attached to the rock by a foot.
Branchial and atrial openings for inlet and outlet of water respectively.
Atrial cavity becomes more extensive.
Pharynx becomes enormously enlarged with a large number of stigmata, forming branchial sac, for filter feeding.
Digestive system becomes well developed: Stomach enlarges, intestine elongates and liver develops.
Test spreads to cover the entire animal, becomes thick, tough and vascular. Attaches the animal by forming a foot if necessary.
Circulatory system with heart and pericardium develops.
Gonads and Gonoducts appear from Larval mesodermal cells.
Affinities
Similarities with Non-chordata
Porifera, Coelenterata are also sessile.
Filter feeding and respiration through a water current like in sponges and molluscs.
Colonial mode of life of simple and composite fixed ascidians.
Similarities with Chordates
Ascidian tadpole larva has all the basic chordate characters: Rod like Notochord forming axial skeleton of tail, dorsal tubular nerve cord, gill slits in the pharyngeal wall.
Similarities to Hemichordata Same structural plan of Pharynx perforated by gill slits and having similar accessories.
Similar development of central part of neural system.
Restricted notochord.
Differences with Hemichordata Balanoglossus lives in burrows, but Urochordates may be fixed.
Division into proboscis, collar and trunk not present in Urochordata.
True notochord in tail of Ascidian larvae, unlike buccal diverticulum of Balanoglossus.
Affinities with Cephalochordata
Ciliary filter feeding — food concentration and respiratory mechanism.
Large pharynx with similar accessories.
Endostyle and associated parts.
Atrial complex.
Identical early stages of development.
Tail with median vertical fins without fin rays.
Median sensory organs – otocyst, ocelots, statocyst.
Affinities with Vertebrate
Dorsal tubular nerve cord.
Axial skeletal notochord.
Pharyngeal gill slits.
Post Anal tail covered by vertical caudal fin.
Cleavage and gastrulation pattern.
Neural gland is homologous with vertebrate pituitary, endostyle to thyroid.
Tadpole larva represents the relic of a free swimming ancestral chordate.