Barreleye fish shows a transparent head..

Macropinna microstoma (and other fish in the family Opisthoproctidae)

The barreleye in the video above is one of the few ever seen alive with its distinctive transparent head. Although it has been documented since 1939, in early specimens nets or lines had pierced and deflated the fluid bubble that makes this fish so unique.

The transparent head works like a cockpit in a fighter plan. The fish can rotate its eyes backwards and upwards to see prey and predators. Useful, eh? It also is one of the few animals on earth that can put its anatomy on display without dying.

Too bad it dwells in such deep water, or else I'd buy an aquarium for one of those things with my HubPages earnings.