Jorunna are colorful sea slugs with what appear to be a puffy tail and rabbit ears (actually rhinophores). They have many variations in color and markings.
Some seas slugs like these purple sea hares can be two feet long, but the little ones are a popular aquarium addition, especially in Japan.
Paste this into Google Image Search to see lots of these little cuties:
ゴマフビロードウミウシ
Here are a couple from Flickr’s Creative Commons:
Via Steve Childs:
Via Sylke Rohrlach:
