So this creature's lungs are on the outside. These gills can be quite elaborate and fluffy... almost like a five year old ballerina's tutu.
But the bottom line is that a nudibranch is a sea slug. A very beautiful, colorful, patterned sea slug. It's a snail without a shell.
But to me (and many others), it's so much more. They are like the Butterflies of the Ocean.
Neville Coleman said it well in his book 1001 Nudibranchs:
"The fun is in the finding."Adding to the unknowns and extending the known existence of species diversity and knowledge is what keeps most nudibranchers enthralled and those that get "bitten" are usually "hooked" for life.
However, let it be known that if after your dive, your divemaster tells you that you brought up a "nudibranch", it usually means you have a slimey booger on your face. Rinse thoroughly before exiting the water. ;)