The shape of the tip of the hectocotylus has been much used in octopus systematics. The oral surface of the modified region of the hectocotylus is wide, swollen, and fleshy. Males also have a well-developed hectocotylus on the third arm. For hectocotylized arms see hectocotylus variability. One arm in the male is modified into a hectocotylus. The male has the fourth arm on the left modified into a hectocotylus. The fourth left arm of males is a hectocotylus. The hectocotylus develops from the left or right fourth arm. One of the ventral arms develops into a secondary sexual organ (the hectocotylus) in males. The males use a modified arm, the hectocotylus, to transfer sperm to the female. The males of most cephalopods develop a specialised arm for sperm delivery, the hectocotylus. Though not all species use a hectocotylus; for example, the adult nautilus releases a spadix. Fertilization occurs when the hectocotylus is deposited from the male in the female's mantle cavity. The hectocotylus when found in females was originally described as a parasitic worm.