Calcareous sponges
The lemon sponge is a lemon-colored calcareous sponge that can obviously be seen in the ocean floor. This sponge grows in sacs, which can have an unusual, lobed shape. Like other members of the phylum Calcarea, the mineral skeleton of the lemon sponge mostly is made up of calcium carbonate. Like all sponges, the lemon sponge draws in and filters water from its collar cells and out its oscullae (the large holes on the sponge's body).