Sexes are separate but males and females are alike showing no sexual dimorphism.
Gonads:
The gonads, either testes or ovaries, lie in the ventro-lateral part of the body wall beneath the myotome, projecting into the atrium. There are about 26 pairs of testes or ovaries arranged metamerically, i.e., one pair in each of the segments 25 to 51. They begin from about the middle of the pharynx and run up to the atriopore on each side. Each gonad is a simple hollow sac and mesodermal in origin. Gonads are derived from the walls of the coelom and each is enclosed in a pouch of the coelom, the gonocoel.
Gametes:
In sections, ovaries have cells with large nuclei but testes present a streaky appearance. The mature sperm of Branchiostoma is about 18 μ in length having a nucleated head with acrosome, a middle piece and a long tail. The ovum is spherical about 0.1 mm in diameter with a large nucleus. It is microlecithal type.
Gonoducts:
There are no genital ducts. When gametes mature the walls of the gonads burst and ova or sperms escape into the atrium from where they escape through the atriopore.
Development:
Development in Branchiostoma is indirect, i.e., having a larval stage. Fertilisation is external taking place in sea water.