Spinous injury caused by a sea urchin.

A bather on holiday in Kenya injured a finger on a spiny marine creature living on the sea bed. A skin biopsy specimen from the injured finger contained several black spines about 0.5 mm in diameter and up to 1.5 cm in length. Spines removed from the specimen were embedded in plastic resin to facilitate transverse sectioning. Light microscopical examination using crossed polarisers showed an ornate symmetrical structure brightly illuminated against a dark background. These features are characteristic of sea urchin (Echinoderm) spines which are composed of ornately formed calcite crystals covered by an epithelium. The spines of sea mice, on the other hand, are chitinous in nature; they are also much finer and lack the ornate symmetry of sea urchin spines.