Simplify the following text for a Grade 4 native English-speaking person.
Original: In one large group of sea slugs , the gills are arranged as a rosette of feathery plumes on their backs , which gives rise to their other name , nudibranchs .
Simplified: Their gills are often in a form of feathery plumes on their backs . This is what gives the name to the nudibranchs .