Chapter III-A. Offices and Ministries of Holy Orders
59. Every authentic celebration of the Eucharist is directed by eh bishop, either in person or through the presbyters, who are his helpers. 
Whenever he is present at a Mass with a congregation, it is fitting that the bishop himself preside over the assembly and associate the presbyters with himself in the celebration, if possible by concelebrating with them.
This is done not to add external solemnity, but to express in a clearer light the mystery of the Church, which is the sacrament of unity. 
Even if the bishop is not the celebrant of the Eucharist but assigns someone else, he should preside over the Liturgy of the Word and give the blessing at the end of Mass.
60. Within the community of believers, the presbyter is another who possesses the power of orders to offer sacrifice in the person of Christ.  He therefore presides over the assembly and leads its prayer, proclaims the message of salvation, joins the people to himself in offering the sacrifice to the Father through Christ in the Spirit, gives them the bread of eternal life, and shares in it with them. At the Eucharist he should, then, serve God and the people with dignity and humility; by his bearing and by the way he recites the words of the liturgy he should communicate to the faithful a sense of the living presence of Christ.
61. Among ministers, the deacon, whose order has been held in high honor since the early Church, has first place. At Mass he has his own functions: he proclaims the gospel, sometimes preaches God's word, leads the general intercessions, assists the priest, gives communion to the people (in particular, ministering the chalice), and sometimes gives directions regarding the assembly's moving, standing, kneeling, or sitting.