The Leptocephalus (which means slim head) is actually a the flat and transparent larva of the eel, marine eels, and other members of the Superorder Elopomorpha.
The group evolved in the Cretaceous, 140 million years ago, and they all have laterally compressed bodies that contain transparent jelly-like substances on the inside of the body and a thin layer of muscle with visible myomeres on the outside.