The Ricketts description set me off looking for information about Membranipora membranacea. It is a fascinating little colonial animal. Here are some notes and a sketch.
By Robert H. Morris, Robert Hugh Morris, Donald Putnam Abbott, Eugene Clinton Haderlie
Chapter 6 - Bryozoa and Entopracta : The moss animals
John D Soule, Dorothy F Soule and Donald P abbott
"The bryozoa, or Ectopracta are virtually all colonial forms, with each colony (zoarium) composed of many small attached individuals (zooids). A colony originates from a single, sexually produced individual (the ancestrula) and increases by asexual budding of new individuals. Bryozoans are widely distributed in the sea and many are found on rocky shores that are exposed at only the lowest tides, or on harbor pilings or ships hulls. At first glance some colonies may be mistaken for bushy types of hydroids brancing corals or marine algae.
The individual zooids in a bryozoan colony are usually less than 1mm long and each is encased in a secrteted outer cuticle or exo-skeleton, which stiffens the colony and provides support and protection for the enclosed soft parts. "

