Zouri-bugs?

I'm wearing zouri today. My intent was to take out and properly store all the new kimono items I bought in New York a few weeks ago, for Tankoukai—the Philadelphia Urasenke tea group's official induction as a chapter of Urasenke Kyouto—but the zouri are so comfortable, with their tatami insoles and rubber soles, that I can't take them off.

Anyway, I was looking up the kanji for zouri—草履, "grass shoes"—and Denshi Jisho randomly mentioned that if you add mushi 虫 ("bug") to zouri you get a paramecium. Why on earth is a paramecium a zouri-bug? Cさん suggests it's about the shape, but I doubt that, as zouri are almost rectangular. The need for further research is clearly indicated.

古池に草履沈みて
霙かな
furu ike ni zouri shizumite
mizore kana
a sandal sinking in the old lake—sleet!
—Buson

(Texts tend to translate "哉" as an exclamation point, expressing wonderment, so that's what I've done here. To me, though, it never seems quite appropriate. It's hard to see Buson getting so jazzed about a frozen sandal, and at any rate modern "なぁ" seems more like "hmm".)