Both declensions have an "unmarked" case, where the word takes no ending in the plural and a -i in the singular. This is true even of second declension even though the endings are otherwise very different because in the before-times, in the long-long ago, extreme Abasement in speech involved using second declension endings to refer to yourself. There's some linguistic history there I have not yet uncovered. First Declension (People Only) Second Declension (everything else)