Fertilization is external. Germ cells do not ripen in the gonads nor are gametes stored here so the gonads are never large. Developing germ cells are released into the perivisceral coelom where they complete their maturation and become spermatozoa or ova. Mature gametes accumulate in the metanephridia prior to release. They are then forced out the nephridiopore by contractions of muscles in the nephridial wall.
Spiral cleavage produces a trochophore larva.
References Andrews, E. A . 1890. Notes on the anatomy of Sipunculus gouldii Portuales. Johns Hopkins Univ. Brown, F. A . Selected Invertebrate Types .