Given that all sponges are filter feeders, why does it follow that all sponges are aquatic?
It does not follow. Granted, all sponges are aquatic and all are filter feeders. But the one does not prove the other. Being a filter feeder does not mean a creature has to be aquatic. The web building spiders could be called filter feeders, or at least filter trappers or hunters. The closest I can come to saying it follows (and this is what your teacher probably wants to hear) is that for a sessile ( non-moving) creature to get enough nutrients as a filter feeder it needs to live in a nutrient rich medium, it also needs to live in a place where that medium moves enough to bring more nutrients to it. On Earth, the air certainly moves as much as the water but the water is much richer in nutrients, small organisms, plant life etc. than the air. But one can imagine a creature that requires very little nutrients and watersand that has a very large filter area that could filter small insects or even micro-organisms out of the air. say something as thin as a spider web but the size of a bed
