 The unusual genus , <unk> , is endosymbiotic , being found only in the oesophagus of sea cucumbers . It has mantle folds that completely surround its small valves . When the sea cucumber sucks in sediment , the bivalve allows the water to pass over its gills and extracts fine organic particles . To prevent itself from being swept away , it attaches itself with <unk> threads to the host 's throat . The sea cucumber is unharmed . 
