kind of snail with spiral shell
螄 refers to a specific kind of snail characterized by a spiral shell. The meaning is narrow and concrete, centered on that particular mollusk.
螄 combines 虫 (insect/creature) with 師, which likely contributes the sound. The character is used for a snail, a small creature, but the exact historical development is uncertain.
The left side 虫 signals a small creature, and the right side 師 looks like a teacher with a turban. Imagine a snail teacher with a spiral shell on its head, slowly lecturing about spiral shells.
For シ, picture the snail teacher saying 'she' (シ) as it points to its own spiral shell: she -> シ.