Parasitic association between Stilapex (gastropod mollusc) and Scotoplanes (holothurian ‘sea pig’).
The arrow shows a couple of molluscs deeply (and happily!) embedded in the holothurian skin (sea cucumber), on its ventral side (close to the abdomen). The molluscs emerge only with less than half their shell and live permanently in this position, never leaving the shelter. They feed on body fluids of the holothurian, by using the long and extensible proboscis which is an elongated appendage from the head of an anima (right bottom).