how can i access "Thing" class ???

Question

In an ontology?

i have an ontology that it doesnt have only one root class.it has 4 seperated classes(4 sub graph) ---1---- with listclasses() method i can't access 3 of them. this method lists one root class and its subclasses!

i want to get all classes of ontology.then i want to get "Thing" class to access all of classes!!! is this possible ??? -----2---- how can i compute distance between 2 classes?

Thanks.