Greenish -
yellow in color and growing only as large as the shell they live in, The Boring Sponge lives in calcium rich objects.
Through a chemical process they "drill" tiny holes in shells. They are important because they enable
calcium that is in solid form to be changed into a calcium solution. Which can then be used to build other structures
http://www.vattenkikaren.gu.se/fakta/arter/spongiar/clionida/clioe.html
For more information see