Peristerium

A peristerium, or Eucharistic dove in the Western church, is a metallic vessel, in the shape of a dove (pigeon), which can be hung over an altar, most often in Eastern Christian churches.

These consecrated Hosts are in the Pyx, which could be put into it by a flap in the back of the dove's body.

German theologian Matthias Joseph Scheeben wrote of the significance of using a peristerium to hold the Eucharist in The Mysteries of Christianity: "How striking and well devised was the ancient usage of reserving the Eucharist in a receptacle symbolic of the Holy Spirit in a vessel fashioned in the form of a dove — in the so-called peristerium!

How beautifully the Holy Spirit was thus symbolized as He who brings and fashions the gift contained in that receptacle; as He who, encompassing and permeating that gift as fire does the coal, dwells therein with His essence and His power!

It symbolizes the Holy Spirit, is an attribute of the Virgin Mary, and was later one of the apostles.