ClipArt ETC
Calcolynthus primigenius, a simple sponge with the wall removed to show the inside. It is hollow, attached at the base with an opening at the top. There are many small holes though the wall with water constantly entering and passing out through the mouth at the top.

Sponge

Calcolynthus primigenius, a simple sponge with the wall removed to show the inside. It is hollow, attached…