Sacoglossan sea - slugs suck the sap from algae , using their one - row radula to pierce the cell walls , .
Dorid nudibranchs and some Vetigastropoda feed on sponges and others feed on hydroids .
Sacoglossan sea - slugs suck the sap from algae , using their one - row radula to pierce the cell walls , whereas dorid nudibranchs and some Vetigastropoda feed on sponges and others feed on hydroids .