Retrogressive metamorphosis is characteristic of ascidians (Urochordata or Tuncicata). It is given this name because here a progressive, active and alert larva metamorphoses into a retrograde and sedentary adult. Ascidians are marine, sedentary, bag-like creatures that remain attached to a rocky substratum all their lives and use a current of water passing through their large and perforated pharynx to collect microorganisms on which they fads. The larva, on the other hand, is active, tadpole-like in shape and has all the three chordate features, gill clefts, notochord and dorsal tubular nerve cord. Most of these characters are lost or become degenerate as the active larva metamorphosises. Hence, the term retrogressive metamorphosis.
Herdmania pallida (Rhabdosynthia) is the Indian ascidian which has been studied fully. It is commonly called sea squirt because it squirts out a jet of water when disturbed. It is fixed to a rocky surface by a broad foot and is about 9 cm in diameter. The sac-like body has two apertures, an anterior branchial aperture and a dorso-lateral aperture at the free end of the body. The water enters through the branchial aperture and leaves the body by the atrial aperture, via the pharynx and atrial chamber. This current of water brings with it microorganisms that stick to the mucus secreted by the pharynx and form the food of the animal. There are two coverings for the body, an inner delicate and muscular mantle, hanging down into the body cavity and attached only at the rim of the two apertures and an outer, thick and leathery tunic composed of cellulose. The alimentary canal has a large branchial sac or pharynx fused with the mantle along the anterior and ventral sides, where there is a ciliated groove called the endostyle which secretes mucus and catches food material. The rest of the alimentary canal has a simple stomach and intestine which opens into the atrial cavity. There is a simple, elongated heart enclosed in a pericardium and simple vessels at either end. The blood changes the direction of flow alternately. The gonads, both ovary and testis, open into the atrium. Fertilisation is external in the sea water.
Development includes a holoblastic and unequal cleavage and a oeloblastula with micro and megameres. Gastrulation is by invagination and the embryo elongates into a tailed-ascidian tadpole larva with an oval body and laterally compressed tail. It posseses all the chordate characters including a sensory vesicle. It moves about actively for some time and then starts its retrogressive metamorphosis.
The larva becomes attached to a hard substratum by three papillae, with the tail up and the following changes take place:
1. The tail is gradually resorbed along with the fin, muscles, notochord and nerve cord, by the process of phagocytosis.
2. The ventral region between the mouth and papillae grows so much that the shortening tail is pushed from the top to a side and the atrial aperture is brought to the top.
3. The outer covering or tunic or test becomes thick and develops a broad foot below.
4. The atrial cavity and the pharynx enlarge in size and almost fill up the entire body cavity. The pharynx becomes perforated by developing stigmata or gill slits.
5. The central nervous system degenerates and the brain becomes solid and small (neural ganglion) between the two apertures.
6. The papillae and tail disappear. So do all chordate characters, except the pharyngeal slits.
7. Gonads are developed and the metamorphosis is completed.