1. Rav Huna: One may write Tefilin on the skin of a Kosher bird.
2. The Gemara is unsure about whether one may write Tefilin on the skin of a Kosher fish.
3. The Gemara explains how we know that one may write Tefilin only on the hide of a Kosher animal (not excluding #1 and #2).
4. The Gemara explains the source for how we know on which part of the body to perform a Bris Milah.
5. One may not make a large mixture of saltwater on Shabbos.
A BIT MORE
1. Although there are holes in the skin where the feathers were removed, the holes are very small and the ink passes over them, and the holes will not be seen.
2. This is not a matter of whether or not a fish has skin, as it certainly does. Rather, the question is whether their Zuhama ceased. (See Me'iri and Chidushei ha'Ran for different explanations.)
3. The verse states with regard to Tefilin, "In order that the Torah of Hashem will be in your mouth." We derive from here that Tefilin may be written only on an animal that is permitted to go into a Jew's mouth (i.e. it must be a Kosher animal).
4. The word "Orlah" is used both in reference to fruit and to Bris Milah. Just as Orlah with regard to fruit refers to procreation, Bris Milah refers to a place of procreation.
5. This is because it is akin to tanning hides (which is a Melachah).