The central apse of the Euphrasian Basilica is furnished with a semicircle marble bench for the clergy in the middle of which there is the bishop’s throne simple in shape, adorned only with the dolphins carved at the end of the bench symbolising resurrection. Above the bench and the throne, there is a symmetrical array of rectangular panels executed in the precious opus sectile technique of decoration.
It is an inlay composed of polished marble tiles in many different hues, tiles made of glass paste and mother-of-pearl, and occasionally of ivory. The area of the sanctuary is walled off by a marble railing of which the fragments, a number of whole pillars, and a few stone panels have been preserved. In 1932, it was restored to its approximate original shape. The apses’ walls were covered to the last centimetre with the mosaic, of which only the one in the central apse has been preserved depicting a complex theological and symbolic image. At the very top, Christ is depicted with an open book in his hand, with six apostles to his sides. The apse calotte contains a complex scene depicting the Virgin sitting on the throne with Christ on her lap. There are three persons placed amongs the saints, but only one of them has a halo around his head. Those are the representatives of the Parentine church. The figure with the crown and the halo is St. Maurus, the Parentine patron saint, who acts here as an intercessor of the earthly Church of Parentium before the divine Christ and Mary. The person with an imposing face, short bearded, and dressed in a purple robe is Bishop Euphrasius, holding a model of his basilica, extending it toward the Virgin. Beside Euphrasius, there stands Archdeacon Claudius, the person in rank next to the bishop who, given his position, was probably in charge of the technical building and furnishing of the church. Between Claudius and Euphrasius a small figure is painted, described as Euphrasius, the son of Archdeacon Claudius.
Along the edge of the inner side of the triumphal arch, there is a series of medallions. The top one depicts Christ as the “Lamb of God”, inserted during the restoration at the turn of the 20th century, while the other medallions present the portraits of saints. There is an inscription beneath each of them, which indicates that some of them were very popular and venerated in Istria.
The careful scientific research carried out in the 1990s, during which the entire mosaic surface was analysed tile by tile, has shown that more than 80% of its surface is authentic. This means that, except for the gilded surfaces, all of the presented figures and most of the decorations and images have retained their structure, and that some of the polychrome tiles occupy the same position as in the time when they were placed – in the mid-6th century.
The large ciborium in the centre of the sanctuary visually fits perfectly into this space, although it does not date from the 6th century as the rest of the furnishing and decoration. It was inserted in 1271. The church must have had a ciborium before that time and the present one is perhaps the copy of the older one; this assumption is supported by the four capitals that date from the 6th century and were probably part of the previous ciborium. The capitals were carved in a very similar way to those in the basilica’s arcature. The upper part of the ciborium, which consists of four slightly pointed arcades, was created by the Venetian masters, who were at that time also furnishing the interior of St. Mark’s Basilica in Venice with polychrome marble, in a rather similar manner. The Venetian artists also made the ciborium’s mosaics. Stylistically, they are comparable to the mosaics in some of the domes in the narthex of St. Mark’s Basilica.
The present altar dates from the 17th or 18th century and contains a silver and gilded altar frontal – antependium in its front, commissioned in 1449–1454 by Bishop Johann. Unfortunately, it was found in a rather damaged state, because its main parts were stolen twice. The first time, in 1699, the reliefs presenting the saints were torn out and had to be replaced by new ones, which were made in the then fashionable Baroque style. Those Baroque reliefs were stolen in 1973 and have not been found to date. All that we are left with is the original decorated frame.