What is the difference between these two compounds:

(Z)-5-Ethyl-2,4,6-trimethyl-2,4-octadiene
(Z)-5-Ethyl-2,4,6-trimethyloctanadiene

Which is the correctly formed IUPAC name?

What does the "na" mean in "nadiene"?
Why does the latter name, not specify the position of the double bonds?