Dictionary of Ichthyology  

leptocephalus = the transparent ribbon- or leaf-like larvae with large teeth and a far-back anus in certain primitive Teleostomi, e.g. Congridae, Anguillidae, Albulidae, Elopidae and Notacanthidae. The larvae first grow, then shrink prior to metamorphosis.

Conger eel leptocephalus (above) and drawing of a leptocephalus from Wikimedia Commons: http://commons.wikimedia.org/wiki/File:LeptocephalusConger.jpg and http://commons.wikimedia.org/wiki/File:Leptocephalus.jpg

Canadian Museum of Nature