How do I make a dichotomous key out of these 9 animals?

I need to make a dichotomous key of these 9 animals: sea urchin, sea cucumber, brittle star, bony fish, dogfish shark, lizard, toad, chicken and pig. I can't start with a vertebrate-invertebrate grouping because according to my teacher, it's been overused now. I've been thinking of grouping them into endotherms and ectotherms, but I'm stuck there. Please help me.|||Of course you know there are an infinite number of ways to do this, right? The object is to divide them into two groups, then subdivide each group until you isolate each creature. You could use things like the presence of lungs in the adult, scales, bilateral and/or radial symmetry, circulatory system. You have a great idea for a start, so go do it. Good luck.|||Tell your teacher to stick his/her opinions where the sun don't shine. The vert./invert. dichotomy is "overused" because it is the natural one. If you don't want to do that (or do want to pass the class), why not divide them into whether they are aquatic or terrestrial as adults? Then you can separate out the echinoderms from the fishes on the basis of symmetry (radial vs bilateral). Or just start with symmetry.