Convergent evolution occurs when species occupy similar ecological niches and adapt in similar ways in response to similar selective pressures. For example, Dogfish (Pisces) and whale (mammas) have acquired aquatic character though distantly related.
Rat and dog (mammals), bacterium and protozoan (unicellular), and starfish (mollusc) and cuttlefish (Echinodermata) show divergent evolution.
Related Theory
Sea stars do not have gills, scales, or fins. Sea stars live only in saltwater. Sea water, instead of blood, is actually used to pump nutrients through their bodies via a 'water vascular system. Hence, they are not considered as fish.