Nudibranch translates to “naked gills”. For dorid nudibranchs like this Mexichromis
mariei, their gills are in the shape of a pompom on their backs, whereas other
clades of nudibranchs have gills called cerata that look like shag carpets on the sea
slugʼs back.

Sketch and fact by Karli Chudeau

Source: Hancock, A., & Embleton, D. (1852). XIII. On the anatomy of Doris. Philosophical Transactions of the Royal Society of London, (142), 207-252.