The free-swimming life of the larva is very short as it is without any arrangement for feeding. It swims for a few hours or so or even almost for a day, then becomes sluggish, sinks to the bottom and attaches itself by the anterior adhesive papillae or “chin warts” as they have been called by some authors. For some time it stands erect, head down and then enters into a phase of metamorphic changes, in which some already differentiated tissues are lost and some undergo differential growth. Such a metamorphosis is called retrograde or retrogressive metamorphosis, Very rapid increase of test takes place. The entire tail with its notochord is partly withdrawn and absorbed by phagocytosis and partly it is cast off in shreds.
The posterior part of the nerve tube disappears with the tail, and the anterior end with its promising little brain degenerates. The remaining part is reduced to form a small ganglion of the adult, underneath which the neural gland is formed. With the degeneration of the locomotory, nervous and sensory organs there is a progressive growth of the digestive and reproductive organs. The branchial sac enlarges, gill-slits multiply in number and the atrium is extended. The stomach and intestine develop as an out-growth from the back of the branchial sac at the side. The region of the body between the point of fixation and the mouth, which is morphologically ventral, increases rapidly in extent and the true dorsal region stops growing and degenerates. The result is that the body of the growing adult rotates through 180 degree and the branchial siphon is carried to the end opposite to that of attachment. Thus a degenerate, sedentary fixed adult is formed with little or no trace of chordate characteristics so marked in the larval stages. This degenerate adult has been described by some authors as “an animate food-sieve with necessary accessories for digestion, circulation, excretion of wastes, and just enough brain to regulate the activities” (Newman).
The changes involved in the retrogressive metamorphosis are:—
1. Increase in the number of pharyngeal stigmata.
2. Diminution and eventually complete disappearance of the tail with the contained notochord and the caudal part of the nerve cord.
3. The disappearance of the sense organs.
4. Dwindling of the central part of the nervous system to a single ganglion.
5. Formation of the reproductive organs.