"Severe granulomatous arthritis due to spinous injury by a ""sea mouse"" annelid worm." A case of destructive arthritis and soft tissue granulomatous inflammation occurred in a 25 year old man who had injured his right index finger while snorkelling in the Mediterranean. It was initially thought that he had fallen on a sea-urchin. He removed some spines at the time of injury but the finger became stiff, swollen, and painful, and after eight months with no symptomatic improvement amputation through the proximal phalanx was performed. Examination showed an exuberant granulomatous and foreign body type inflammation in the dermis and subcutaneous tissues and affecting the bone, with erosion of the cartilaginous surfaces of the proximal interphalangeal joint. Spines present in soft tissue sections contained no calcium but did contain chitin as shown by a von Wisseling reaction for chitosan. It is concluded that the chitinous spines almost certainly came from a sea-mouse (Phylum Annelida, family Aphroditidae). Sea mice are inconspicuous creatures which live on the sea floor and which may cause some injuries thought to be attributable to sea-urchins.