have markedly different reproductive strategies, with colonial forms having mixed modes of reproduction. Solitary ascidians release many eggs from their atrial siphons; external fertilization in seawater takes place with the coincidental release of sperm from other individuals. A fertilized egg spends 12 hours to a few days developing into a free-swimming tadpole-like larva, which then takes no more than 36 hours to settle and metamorphose into a juvenile. As a general rule, the larva possesses a long tail, containing muscles, a hollow dorsal nerve tube and a notochord, both features clearly indicative of the animal's chordate affinities. But one group, the molgulid ascidians, have evolved tailless species on at least four separate occasions, and even direct development. A notochord is formed early in development and always consists of a row of exactly 40 cells. The nerve tube enlarges in the main body, and will eventually become the cerebral ganglion of the adult. The tunic develops early in embryonic life and extends to form a fin along the tail in the larva. The larva also has a statocyst and a pigmented cup above the mouth, which opens into a pharynx lined with small clefts opening into a surrounding atrium. The mouth and anus are originally at opposite ends of the animal, with the mouth only moving to its final (posterior) position during metamorphosis. The larva selects and settles on appropriate surfaces using receptors sensitive to light, orientation to gravity, and tactile stimuli. When its anterior end touches a surface, papillae (small, finger-like nervous projections) secrete an adhesive for attachment. Adhesive secretion prompts an irreversible metamorphosis: various organs (such as the larval tail and fins) are lost while others rearrange to their adult positions, the pharynx enlarges, and organs called ampullae grow from the body to permanently attach the animal to the substratum. The siphons of the juvenile ascidian become orientated to optimise current flow through the feeding apparatus. Sexual maturity can be reached in as little as a few weeks. Since the larva is more advanced than its adult, this type of metamorphosis is called 'retrogressive metamorphosis'. This feature is a landmark for the 'theory of retrogressive metamorphosis or ascidian larva theory'; the true chordates are hypothesized to have evolved from sexually mature larvae. Direct development in ascidians Some ascidians, especially in Molgulidae family, have direct development in which the embryo develops directly into the juvenile without developing a tailed larva.