Why didn't Sirius apparate?

[sylviablundell2001]

No: HPFGUIDX 82487

Many thanks, Melinda and Tom, for your explanations, which I will 
pass on to my own Tom.  I think Tom (Wall) is probably right that in 
order to apparate, Sirius would have to turn back into a man, which 
would signal to even the dimmest Dementor that he was escaping. , I'm 
not quite sure about the mechanics of apparating. Is it necessary to 
know or to have visited the place you are apparating to? This doesn't 
seem to apply when using the Floo network, where it is sufficient to 
say the name of the place you wish to go to, but apparating is 
probably a lot more complex.
Sylvia (who quite understands that Sirius would not want to appear in 
the middle of Sainsbury's.)