Externally the sexes are usually alike and separate. Gametes (mature germ cells) are shed into the body cavity and collect in nephridia (excretory organs) that become modified as egg- and sperm-storage organs; they are emitted into the sea from nephridiopores. Fertilization takes place outside the body. The trochophore (free-swimming) larva, which results from spiral cleavage of the zygote (the cell formed by the fusion of two gametes), undergoes metamorphosis to its characteristic shape.