Xenophyophores - That's one wierd looking single-celled organism

Single-celled organisms are generally required to maintain microscopic sizes. Xenophyophores, immobile shell-making mud-stickers, however, brazenly ignore all requirements of general microbial decency by attaining sizes not merely macroscopic, but positively enormous (at least by unicell standards). One of the largest species, Stannophyllum venosum Haeckel 1889, is a broad flat form up to 25 cm across, although only about a millimetre thick.  Tendal (1972).

Despite such impressive dimensions, mention of them is likely to garner blank looks from most of the general public, and even from many biologists who probably should know better. This is because xenophyophores are restricted to the deep sea, not usually regarded as a prime holiday destination.  Those that are occasionally pulled up from below are probably not recognised.  Like benthic Steptoes, xenophyophores surround themselves with all sorts of junk they find lying around, which they use to make their shells, stuck together with a cement of polysaccharides.  Id.  Foraminiferan and radiolarian shells, sponge spicules, mineral grains – all are potential building materials (though individual species are often quite picky with regard to exactly what they use, and some species eschew foreign particles altogether). The particles used are referred to as xenophyae.  When the fragile test is brought up, these particles tend to all fall apart, and are hence not recognised as having once been part of a larger whole.
Image: Syringammina from the web page of J. Alan Hughes.

http://www.newscientist.com/article/dn18468-zoologger-living-beach-ball-is-giant-single-cell.html?DCMP=NLC-nletter&nsref=dn18468
http://www.palaeos.com/Eukarya/Units/Rhizaria/Xenophyophorea.html