(2) Pre-larval development: After fertilization zygote undergoes cleavage. Cleavage is holoblastic and radial, results into morula. After 6-15 hrs of fertilization morula develops into coeloblastula having fluid filled cavity blastocoel.
In next stage of embryo, the gasturla, archenteron develops which opens outside through blastopore. The gastrula now lengthens and blastopore gets closed. The tip of archenteron is pinched off as a coelomic vesicle called the protocoel, and the remaining portion of archenteron marks the future gut. (3) Larval Development: Remaining inside the egg, embryo grows in size, gut further develops completely. The embryo becomes uniformly ciliated larva and escapes from the egg membrane to lead a free swimming larval life. It is called tornaria larva. Tornaria larva: It is clear, glossy in appearance with an oval body ranging upto 3.0 mm in size.
It is so called because of its habit of rotating in circles. Larval gut is complete, starts from mouth to anus. Cilia are found in two bands. Anterior ciliary band or circumoral band takes up a winding course over the preoral surface and forms a postoral loop. The cilia of this loop are short and serve to collect food.
Posterior ciliated band or telotorch occurs as a ring in front of the anus. Its cilia are long and serve as locomotor organs. At the anterior end is an optical plate of thickened epidermal cells which bears a pair of eye spots or ocelli and a tuft of sensory cilia called apical tuft or ciliary organ. Protocoel (Proboscis coelom) is thin walled and opens outside through a hydropore (proboscis pore). To the right of the hydropore lies a pulsating heart. Collar and trunk coeloms appear in the older larva. Metamorphosis: Larva swims freely leads a planktonic life, feeding on minute organisms and metamorphose into an adult worm.
In adult the ciliary bands get lost and it leads a benthic life.