It is of rare occurrence. It is reported by Gilchrist (1923) in Balanoglossus capensis. During summer the juvenile phase reproduces by cutting off small pieces from the tail end. The separated part later on develops completely into an adult sexual type in next winter.
The sexes are separate but there is no sexual dimorphism. The males are fewer in number than females (1:60). The gonads are similar in size, shape and position. They are paired, branched or unbranched coelomic sacs found in the branchiogenital region. Each sac is lined with germinal epithelium which is continuous with the ectoderm. By the proliferation of germinal epithelial cells sperms and ova are produced. The ripe sperms and ova are shed in water through a gonopore. The ova are larger in size each measures about 1 mm in diameter. The sperms are provided with round head. The acrosome is small but distinct. Fertilization is external.
The breeding season is from May to June. The suitable temperature needed for this purpose is 16°C. About 2000-3000 eggs are laid embedded in a mass of mucous escape from the burrow of the female. After about 20 minutes or so the mass of sperms is discharged that leave the male burrow. Fertilization takes place in sea water. The mucous mass is soon broken by tidal currents and eggs are dispersed
In Balanoglossus, the eggs are microlecithal i.e., contain small amount of yolk. The development is indirect with a Tornaria larva. After about six to seven hours of the fertilization the development starts. The cleavage is holoblastic, equal and of radial type. The cleavage continues for 6-12 hours and results in the formation of hollow blastula or coeloblastula. The coeloblastula is single layered and encloses a central fluid-filled cavity, the blastocoel. Now the blastula flattens along the animal vegetal axis and invagination takes place on the ventral surface. The invagination deepens to form the archenteron. The archenteron opens to outside through a pore, the blastopore. The blastopore soon closed and the embryo now called gastrula. The gastrula now elongates along the antero-posterior axis and becomes ciliated.
The archenteron divides into an anterior protocoel and a posterior gut. The protocoel forms the coelom of proboscis, thus its origin is enterocoelic. As the protocoel is formed the inner end of gut moves towards the ventral side and opens out through the mouth. The closed blastopore is reopened as anus. The gut is differentiated into oesophagus, stomach and intestine. Now the embryo is known as Tornaria larva and it comes out of egg membrane to lead a free swimming life.
The tornaria larva was described by Muller (1850). It has an oval, transparent body measuring upto 3 mm. At its anterior end, it bears a thickened plate, the apical plate which bears a tuft of cilia and a pair of eye spots. The gut is differentiated into oesophagus. stomach and intestine.
The cilia form two bands on the body surface. The anterior ciliary band follows a winding course over most of the preoral surface. It helps in nutrition by directing the water current towards the mouth. The posterior ciliated band or telotroch is ring like in front of anus. It bears large cilia
that help in locomotion. The protocoel is in the form of a thin sac which opens out through hydropore present on dorsal side of larva. To the right side of hydropore lies the heart vesicle. In the older larva the collar and trunk coeloms appear as thin walled sacs in close contact with the stomach.
After swimming for sometimes, the larva sinks down the bottom and metamorphoses into an adult worm. First of all the larva decreases in size and develops an epidermal constriction making the proboscis-collar boundary. Its transparency is lost and the ciliated bands gradually
disappear. The eye spots and apical tuft of cilia are also lost. A second constriction, making the collar-trunk boundary appears. By further elongation and growth, the adult condition is attained. The hydropore persists as proboscis pore. Buccal diverticulum and gill-slits appear as outgrowths of the gut.