S.T First Part, Q. 84, Art. I
macroevolution.net/species.html
Note that Aquinas specifies (no pun

) that the distinctions are ‘sensible’, that is, pertaining to the senses. These are accidental properties.
The Blessed Sacrament retains the sensible qualities of the substance that was confected, that of bread and of wine.
After confection, the Substance is one and the same, with no essential distinction, that of the Body, Blood, Soul and Divinity of Christ.
The accidental, or sensible properties remain, that of bread and of wine, so they remain different species (of bread and of wine) while being of the same Substance.