Nudibranchs are named thus because their lungs or gills (branchs) are outside (nude). In this case the gills are the orange bits to the left on the top of the critter. Like this example many nudibranchs are very colorful and are about 1 ½ long. When at rest one only sees the upper almost flat part with the gills and antennae, but in this picture it is exposing the lower foot which is used to propel the animal. I'm getting more interested these delightful critters because of their pretty colors and designs. See also Green Nudibranch over a Tunicate and Spotted Nudibranch.
This image is available as a square to 10 x 10 and in standard format to 9 x 14.