Nudibranch - Nembrotha lineolatha mating

This nudibranch - Nembrotha lineolatha is also another common species to be found in Philippines.
Nudibranch are hermaphrodite, meaning they have both male and female sexual organs. However, they are not able to fertilise themselves. Their genital is located on the front right of their body and in order to mate, they do a 69 as shown in the picture. Their genitalia is mostly retract when they are not courting/mating but when erect, can be quite long.