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.
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