TA = Saint Thomas Aquinas is known as the Angelic Doctor. He is angelic for three reasons: his purity, his intellect, and his advanced teaching about angels.
Thomas replies that angels are distinct from God. Whereas God’s essence is his existence (God’s essentia = God’s esse), this is not the case for angels. The essentia of an angel is not to exist since angels have a beginning with their creation by God. Angels owe their existence to God. So they are distinct from God in this important way.
This raises a further problem for Thomas Aquinas. If every angel has the same form (angel-ness) then there could only be one angel since there is nothing material to separate them. So, Thomas solves the problem by saying that every single angel has his own unique form. That is, every single angel is his own species of angel. On earth, every human person belongs to the human species, and we are differentiated by our bodies. However, in heaven every single angel is his own species. There is not one single “angelic species.” According to Thomas, if there are one billion angels, then there are one billion different “species” or “forms” of angels. Thus, every angel is formally distinct from every other angel.
Hierarchies Human realm cooperators ongoing process of creation, temporal world unlike angels, all angels created at the same time. Man & woman create new life. Angels do not have this creative capacity. Animals propagate as well, to serve man.
Archangels (Sept 29), The term generally taken to mean “lead, chief, or ruling angel.” (Jude 9; 1 Thes 4:16) They are mentioned throughout Scripture. On a popular level, the term refers to high ranking angels such as Michael the Arch Angel, who within the hierarchy of the nine choirs of angels, is a princely seraph. Just as guardian angles protect, so too the archangel has a unique role as God’s messenger to people at critical times in the salvation process (Tb 12:6, 15; Jn 5:4; Rv 12:7-9)
9 Choirs of angels, office not nature, not species, duty or role they play, essence is spirit
 Taylor Marshall, Thomas Aquinas in 50 Pages, pg 26
 Taylor Marshall, Thomas Aquinas in 50 Pages, pg 27
 Catholic Encyclopedia, Peter Stravinskas ed, (Our Sunday Visitor, Huntington IN 1991) 92
NB Theodore McCarrick has introductory remarks.