The gonads are found in the intestinal region where they form two lateral rows, which are serially repeated, alternating singly or in groups with the intestinal diverticuli. Their shape ranges from spherical to flask-shaped. In most nemertean species, each gonad releases one gamete independently. The exception is in Carcinonemertes. In these nemerteans, "a vas efferens links each testis with a single vas deferens, which in turn leads to a sperm storage chamber (the seminal vesicle) from where the gametes are shed into the rear of the intestine and hence through the anus. (Gibson, 1982)" .
In the ovoviviparous species, the ova are kept in the gonads and internal fertilization takes place. In anoplan species, several eggs are matured at the same time within each ovary, whereas in enoplan nemerteans, one or only a few eggs are matured at a time (Gibson, 1982).
Little is known about the natural reproduction habits of nemerteans. What is known is that fertilization is usually external. Some species like Lineus ruber and Amphiporus lactifloreus have the mature worms come together in a common gelatinous sheath where they discharge their eggs and sperm. The adults exit and leave the fertilized eggs in the sheath matrix. This matrix then adheres to algal fronds or to the surface of rocks. In other nemerteans like the Malacobdella, ripe gametes are shed into the sea and fertilization occurs without the two adults coming into direct contact. In these instances, the stimulus that initiates reproduction is primarily chemical. In a terrestrial nemertean species, the adults deposit gelatinous egg capsules into wet environments (Gibson, 1982).
2.
For what is known, most nemertean species possess a particular breeding season. This period may extend over 3 to 4 months.
Unlike nemerteans, turbellarians have a different set up of reproductive organs. The biggest difference is that both male and female reproductive organs are found in each turbellarian individual.