Male octopuses are distinguished from females by the presence of a modified third arm, the third right arm in most species. This arm has a groove running along the edge and a leaf-shaped grasping structure at the tip. When octopuses mate, the male moves elongate sperm packages, called spermatophores, along the groove to the arm tip. The male places the copulatory arm into the mantle cavity of the female where the tip of the arm inserts spermatophores into the female's oviducts. In some long-armed rock-pool octopuses, the male remains hidden, snaking a long modified arm across the rock pool to transfer spermatophores to the hidden female.