The body of this sponge is orange to yellow in colour and the papillae are pale yellow. The sponge consists of a low, spreading cushion (up to 1 cm thick), with papillae projecting above the sediment covered body. The papillae are differentiated into inhalant and exhalant; the exhalant papillae are large, few in number and open at one end, and can be easily distinguished from the thinner inhalant papillae. This species was previously called Polymastia mamillaris
in Britain. Polymastia mamillaris
is now known to be a distinct species, found in Scandinavia but not currently known from Britain and Ireland. There are several other papillate sponges with which P. penicillus
may be confused including several undescribed Polymastia
species and Ciocalypta penicillus
A common species on western and southern coasts of the British Isles, also reported from the Arctic, Atlantic coasts of Europe, Mediterranean and N. America.