Unio (freshwater mussel), Chiton, and Pila (apple snail) are all molluscs, belonging to the phylum Mollusca. Echinus, however, is a sea urchin, which belongs to the phylum Echinodermata. Echinoderms have radial symmetry, a water vascular system, and tube feet - all features absent in molluscs. Therefore, Echinus is the odd one out because it represents a different phylum entirely.