Here's a pair of clown nudibranchs, captured on film about ten meters down at New Zealand's Poor Knights Islands. These animals were about 1 inch long. The head of the nearest one is to the right. It's gills are sucked back into the body (bottom center of the body). They are nestled head-to-tail.
Although you can't quite see it from this angle, these two were exchanging genetic materials through a pair of tubes - one extended from each animal into the other. Fascinating creatures!