In many leptoid species the medusa stage is not released, and is vestigial to a varied degree. Female medusae in such species retain their eggs, but their tissues are greatly modified to form a protective or supportive gonophore or sporosac. In contrast the sperm from corresponding male gonophores, in most species on distinct colonies, are still liberated into the sea whence they swim to the eggs which are within the female gonophore.

Alternative forms for sporosac : gonophore, gonophores, sporosacs.