The Collaborative International Dictionary of English v.0.48:

Siredon \Si*re"don\, n. [NL., from Gr. ??? a siren.] (Zool.)
   The larval form of any salamander while it still has external
   gills; especially, one of those which, like the axolotl
   (Amblystoma Mexicanum), sometimes lay eggs while in this
   larval state, but which under more favorable conditions lose
   their gills and become normal salamanders. See also
   Axolotl.
   [1913 Webster]