sentences:  Each gonad has a single duct rising from the upper pole to open at a gonopore lying in one of the genital plates surrounding the anus.Some burrowing sand dollars have an elongated papilla that enables the liberation of gametes above the surface of the sediment.The gonads are lined with muscles underneath the peritoneum, and these allow the animal to squeeze its gametes through the duct and into the surrounding sea water, where fertilization takes place.