Prosobranchs Prosobranchs, mainly marine snails, are divided into three Orders. The abalones and keyhole limpets are restricted to hard surfaces such as rocks, because their primitive gills are easily fouled by even small amounts of sediment in the water. Their primitive radula scrapes encrusting vegetation off the rocks. Water is drawn through the new front-most holes into the mantle cavity and gill on the left side, where oxygen is extracted.
Water leaves through the middle holes.
The back holes are the oldest ones and are closed off. Advanced gills are simplified, so fouling is less likely. These snails can thus venture onto the vast sand and mud expanses of the ocean floor. The mouth is elongated into the proboscis. Mantle tissue forms a tubular siphon to inhale water into the mantle cavity.