Martin Gabriel / Woodcock murex snail (Murex scolopax), its aperture locked with operculum, Gulf of Oman, Sultanate of Oman, January, CROPPED

Chris Mattison / Tan and white cowrie (Cribrarula cribraria) shell, showing aperture, on white background.

Olga Kamenskaya / Hole in ice used by Baikal seals (Pusa sibirica) for breathing. Lake Baikal, Russia, April.

Olga Kamenskaya / People looking at hole in ice used by Baikal seals (Pusa sibirica) for breathing. Lake Baikal, Russia, April 2009.

Olga Kamenskaya / Hole in ice used by Baikal seals (Pusa sibirica) for breathing. Lake Baikal, Russia, April.

Olga Kamenskaya / Holes in ice used by Baikal seals (Pusa sibirica). Lake Baikal, Russia, April 2009.

Olga Kamenskaya / Man visible on surface of the ice through dive hole. Lake Baikal, Russia. March 2008.

Pal Hermansen / View to wintry landscape through hole in ice, Hornsund, Svalbard, Norway, September.

Nick Garbutt / Hole chewed in tree trunk by a feeding Aye-aye (Daubentonia madagascariensis) excavating for beetle grubs. In forests near Andranotsimaty, Daraina, northern Madagascar.

Nick Garbutt / Holes chewed in tree trunk by a feeding Aye-aye (Daubentonia madagascariensis) excavating for beetle grubs. In the forests near Andranotsimaty, Daraina, northern Madagascar. Endangered species.

Loic Poidevin / Black woodpecker (Dryocopus martius ) at nest hole, with chicks begging for food, Indre-et-Loire, France, June.

Loic Poidevin / Black woodpecker (Dryocopus martius ) at nest hole, Indre-et-Loire, France, June.

Konrad Wothe / Tarantula (Pamphobeteus antinous) Tambopata National Reserve, Peru, South America.

Konrad Wothe / Tarantula (Pamphobeteus antinous) at hole, Tambopata National Reserve, Peru, South America.

Enrique Lopez-Tapia / DUPLICATE DELETE Icebergs with holes in the ice, Iceberg Alley, Pleneau Island, Antarctica.

Nick Upton / Jackdaw (Corvus monedula) standing at the entrance to its nest hole in a pollarded willow trunk, Gloucestershire,UK, April.

Nick Upton / Jackdaw (Corvus monedula) standing at the entrance to its nest hole in a pollarded willow trunk, Gloucestershire,UK, April.

Alex Mustard / Pygmy cancer crab (Cancer oregonensis) makes its home in a hole within a yellow sponge. Browning Pass, Port Hardy, Vancouver Island, British Columbia, Canada. North East Pacific Ocean.

Alex Mustard / Pygmy cancer crab (Cancer oregonensis) makes its home in a hole within a yellow sponge. Browning Pass, Port Hardy, Vancouver Island, British Columbia, Canada. North East Pacific Ocean.

Markus Varesvuo / Willow grouse (Lagopus lagopus) snow hole, Utsjoki, Finland, April.

Markus Varesvuo / Rock ptarmigan (Lagopus muta) snow hole, Utsjoki, Finland, April.

Philippe Clement / Common whelk (Buccinum undatum) from the North Sea, shell showing aperture, Belgium

Philippe Clement / Common whelk (Buccinum undatum) shell showing aperture, Normandy, France

Philippe Clement / Common whelk (Buccinum undatum) shell showing aperture, Normandy, France

Philippe Clement / Turban top shell (Gibbula magus) showing aperture, Mediterranean, France

Philippe Clement / Pennant's top shell (Gibbula pennanti) with aperture showing, on beach, Normandy, France

Philippe Clement / Turban top shell (Gibbula magus) on beach showing aperture, Mediterranean, France

Philippe Clement / Flat top shell (Gibbula umbilicalis) shell showing the aperture, Normandy, France

Philippe Clement / Trochita shell / Peruvian hat (Trochita trochiformis) shell showing aperture, Chile

Philippe Clement / Rough periwinkle (Littorina saxatilis) shell showing aperture, Normandy, France

Philippe Clement / Netted dog whelk (Nassarius reticulatus / Hinia reticulata) shell on beach with aperture showing, Normandy, France

Philippe Clement / Oyster drill / Sting winkle / Hedgehog Murex (Ocenebra erinacea) shell showing aperture, Mediterranean, France

Philippe Clement / Flat periwinkle (Littorina obtusata) shell showing the aperture, Mediterranean, France

Philippe Clement / Grey top shell (Gibbula cineraria) shell on beach with the aperture showing, Normandy, France

Philippe Clement / Pennant's top shell (Gibbula pennanti) shell with aperture showing, Normandy, France

Philippe Clement / Netted dog whelk (Nassarius reticulatus / Hinia reticulata) shell with aperture showing, Normandy, France

Philippe Clement / Pennant's top shell (Gibbula pennanti) with aperture showing on beach, Normandy, France

Philippe Clement / Turban top shell (Gibbula magus) showing aperture, Mediterranean, France

Alex Mustard / Banded coral shrimp (Stenopus hispidus) shelters inside Pink vase sponge (Niphates digitalis), on a coral reef. Photographed through the narrow aperture of the sponge. Jardines de la Reina, Gardens of...