First, let's start with the gills. Warning: Right now I'm a bit lazy to refer to the book, so I'm making things up as I go along. The gills .... um... I guess they allow them to breathe. There we go!
The horn-like thingies are called rhinophores (not ears).
Nudibranchs do the strangest things... like hang upside down
.... or straddle uncomfortable stones
... or freefall
Above: Emm's Hypselodoris
The Choo-Choo Train (aka trailing behaviour)