Stove pipe sponge (strokes rx / CC BY-ND)

Sponges (porifera) are multicellular organisms that consist of cells embedded in a gelatinous matrix and enclosed by two thin layers of epidermal cells. Sponges lack the digestive, nervous, and circulatory systems known from higher organisms. Complex sponges contain a system of channels, pores, and chambers. The shape of the sponge is supported by tiny structural elements (spicules). Upon contact the Caribbean fire sponge (Tedania ignis) and some other species can cause a painful rash and a severe itching reaction that lasts for days.

More images » More info »

This supplemental online resource accompanies the book Scuba Diving Hand Signals by Lars Behnke, available on Amazon.