BarreleyeThis unusual fish might be the most bizarre creature ever found lurking in the deep ocean. Sometimes called a "spook fish," no doubt because of their strange appearance, the barreleye has a completely transparent head. The purpose of the clear head is that its eyes, which are located inside the head, can look straight up as it swims, presumably so that it can detect the silhouettes of available prey. Its eyes can rotate within the socket to that the fish can look in multiple directions, which would be impossible if not for the transparent cranium. Check out this incredible video of a live barreleye as it swims.

Glasswing ButterflyThis butterfly with transparent wings has a Spanish name, "espejitos," which means "little mirrors." If it wasn't for the opaque outline around the wings, the average observer might not see one perched on a leaf or flower. Adult glasswing butterflies will often migrate great distances, and males of the species are known to lek, or gather in large groups for the purpose of competitive mating displays.

Glass FrogsDon’t worry … you aren't having a flashback to high school biology class. These amphibians of the family Centrolenidae are called glass frogs because the abdominal skin of many species is highly transparent, and viewing them from underneath is like looking at an MRI. Many of the frogs’ internal organs, such as the heart, liver and gastrointestinal tract, are on display. Found in the jungles of Central and South America, these animals are mostly arboreal, meaning they live primarily in trees.

Ghost ShrimpThe semi-translucent shells of these minuscule crustaceans make them nearly as transparent as the glass aquariums in which they are sometimes kept. In the wild, different species can be found in rivers and lakes throughout the world, including the central United States. The animal is so clear in appearance that it only has color after it has eaten a colorful meal, which is typically plant-based and, therefore, usually green.

Glass-like creaturesThere are ghosts currently wandering the planet, but they aren't the undead apparitions of your imagination. Transparent animals — creatures with transparent, glass-like skin — can be found lurking in abundance in ecosystems around the globe. These fascinating, verging-on-invisible organisms are the tangible ghosts of the real world. Here's our list of 11 of the world's most intriguing transparent animals. (Text: Bryan Nelson)

Glass OctopusThis unbelievable octopus is so ghostly and unusual that it occupies its own family, Vitreledonellidae. Little is known about this marine animal, but it can be found in tropical and subtropical waters around the world. Thanks to its transparent skin, scientists know that its optic lobes have unusually long optic nerve stalks, meaning its sense of sight is acute. Your eyesight would have to be pretty good, too, to spot one of these ghosts

Crocodile IcefishThese ghostly Antarctic predators are unusual because their transparent appearance is due in large part to nearly invisible blood. They are the only known vertebrates in the world without hemoglobin, the protein in blood that transports oxygen. They survive without hemoglobin thanks to the subzero temperatures of the ocean where they live, since cold water has a much higher dissolved oxygen content than warmer water.

Tortoise shell beetleThis remarkable beetle is not completely transparent, but it does have a carapace that is nearly invisible. The purpose of the transparent outer shell is to fool potential predators, as it reveals markings on its back that act as a warning. Tortoise beetles come in many different varieties, and the design under their clear shells can be distinct and beautiful.

SalpasNot to be confused with jellyfish, salpas are transparent, free-floating tunicates. Their gelatinous bodies swim by contracting and pumping water through internal feeding filters, feasting while they move. They can be found anywhere, but they are probably most common in the Southern Ocean, where they sometimes form enormous transparent swarms.

Transparent sea cucumberRecently discovered by researchers with the Census of Marine Life, this sea cucumber is so transparent that its digestive tract is on spectacular display. Found at a depth of 2,750 meters, it is one of many unusual finds discovered by a census of the deep sea. When encountered, this cucumber was creeping forward on its many tentacles at about 2 centimeters per minute while sweeping detritus-rich sediment into its mouth.

Glass SquidThere are about 60 different species of glass squid, so-called because many of them appear completely transparent. This transparency keeps them hidden from predators, since they spend much of their lives in partially sunlit shallow waters. Many species are bioluminescent and possess light organs on the undersides of their eyes. Since the digestive gland can still be seen through the transparent skin, it is typically held in a vertical position to reduce its visibility.

JellyfishPerhaps the most well-known types of transparent creatures are jellyfish. Many of the free-swimming members of the phylum Cnidaria are transparent, a trait that makes them occasionally hazardous because of their sometimes deadly stings, which can surprise swimmers. Their translucent bodies also make them among the most elegant and beautiful of the ocean's creatures.