A low, spreading cushion of up to 2 cm thickness with a base diameter of up to 12 cm. The sponge surface is hard and rough with a variable number of erect projections bearing a terminal opening (papillae) that may be as tall as 12 cm. The main body mass is pale, usually greyish yellow or more rarely, orangey yellow, and is often buried in sediment with only the papillae visible. The papillae are smooth and clean, creamy white or pale yellow, sometimes translucent, and often a fine network of spicules can be seen on their surface. The exhalant papillae are large and few in number, the inhalant papillae are smaller.May be confused with Polymastia robusta, however the papillae of this closely related species tend to fuse together, which does not occur in Polymastia mamillaris.
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