ヘビの生殖器は裏返しに脱いだ靴下の様に体内に収納されていて、交尾時には棘(種によって差異あり)を突き立てながらメスの体内に入り込むそうです。(メモ参照)

The reproductive organs of the snake are stored inside the body like socks that have been turned inside out, and it seems that they enter the female body while striking the thorns (with differences depending on species) at the time of copulation. (See note)