So in response to the blue velvet slug, I found this one a website:
"Like all sea slugs, species of Chelidonura are hermaphrodite, with fully functional male and female reproductive systems. In nudibranchs, the male and female organs open together on the right side of the body so for physical reasons only two animals can mate at one time, but when they do so they can act as both male and female at the same time. However in aglajids, like Chelidonura, and in Sea Hares, the male organ - the penis - is situated on the right side of the head, while the female opening is at the posterior end of the body on the right side. This means that when a pair mate, one usually acts as the female, and the other as the male. It also means that if there are more than two animals present they can form a mating chain with the one at the front acting as a female, the one at the back acting solely as a male, but the one[s] in the middle can act as both."