In the main apse, separated by a wooden lattice, Parentium’s most valuable Christian artefacts are displayed. The most important amongst them is definitely the original fragment of the 4th-century floor mosaic depicting a fish. The first letters of the word 'fish' in Greek (ikhthys) were recognized by the Christians as representing the phrase 'Iesous Christos Theou Hyos Soter', which means 'Jesus Christ, Son of God, the Saviour'. Members of the then illegal church used the fish symbol as a secret sign of identification. Next to the mosaic, there is a marble plate from the front side of Euphrasius’ altar base. It contains engravings of symbolic animals, dolphins, and doves. There are miniature doors in its lower part, through which one could see the chamber where the relics were kept. The semi-circular ribbon contains an engraved inscription stating that Euphrasius had the altar built eleven years after becoming a bishop.