Simplify the following text for a Grade 9 native English-speaking person.
Original: Most of the marine limpets have gills , whereas all the freshwater limpets and a few of the marine limpets have a mantle cavity that is adapted to breathe air and function as a lung ( and in some cases has been again adapted to absorb oxygen from water ) all these various kinds of snail are only very distantly related .
Simplified: The phrase `` true limpets '' is used only for marine limpets in the ancient clade Patellogastropoda . This article is mainly about the true limpets .