TT #118 · Parshas Shemini · Aliyah 6 · Passuk 9
English: These you may eat among all that live in water: anything in the water, whether in the seas or in the streams, that has fins and scales—these you may eat.
Bechoros 7b.
E-Daf.com — Bechoros 7b (Tsurras Hadaf)
אֹתָם תֹּאכֵלוּ. לְרַבּוֹת אֶת הַטָּהוֹר שֶׁבְּמֵעֵי הַטָּמֵא, יָכוֹל אֲפִלּוּ טָמֵא שֶׁבְּמֵעֵי הַטָּהוֹר? תַּלְמוּד לוֹמַר אֹתָם.
Textual inclusion and exclusion — “These” includes the pure creature found within an impure creature, while excluding the reverse case.
A pure fish swallowed by an impure fish remains included; the reciprocal case is excluded.