Star Trek Discontinunity - The Trill Problem

Obviously the Trill in TNG look much different from the Trill in DS9 and the subsequent series and movies, both species are a joined race, consisting of a host and a symbiot. However in the TNG Trill, seen in The Host, the symbiot is in complete control of the host’s personality and body. In the DS9 Trill, the bond between host and symbiot is much different, with a completely new personality emerging from that of the symbiot, and current host and any past hosts that a symbiot may have had. Despite the name, these two varieties of Trill are very different, yet obviously are supposed to be the same species. Then why is there such a large difference between there two types of Trill? The real explanation, as far as appearance, is that in the screen tests for DS9 the old Trill make up looked terrible on Terry Farrell, so they designed a new one, with absolutely no regard for how this would adversely effect Star Trek continuity. This offers no explanation, within the Star Trek universe for why there are two completely different types of Trill, so here is one possible explanation: There are two different races of Trill, both living on the same planet. There are also two different races of Trill symbiots each only compatible with its own trill race. We have many different races of humans on earth, so why not on Trill?