The Grass divides as with a Comb - A spotted Shaft is seen,
The speaker goes on to describe what it looks like when you almost step on a snake, and it slithers off.
In a really unique simile, the grass splits like hair being parted with a comb. Now that doesn't seem very threatening, does it?
For being startled by the snake, we think this simile reflects a mind that is more interested in the movement of the snake, than threatened by its appearance.
The snake, described here as a “spotted Shaft,” appears and disappears in the blink of an eye.
Notice the use of passive voice here ("is seen"). It's not clear who is actually doing the seeing, really. Why doesn't the speaker just say that he (or she) saw the shaft? It's as though the poem wants to generalize this experience so that everyone can participate in it. How thoughtful!
And then it closes at your Feet And opens further on –
Aha! It's not even the speaker's feet anymore. It's "your" (meaning ours, the readers') feet. This speaker is really trying to get us to consider the experience from our own point of view, not just his (or hers—we still haven't figured out just who's talking to us).
The grass ("it") closes as the snake moves past it, then opens further away to indicate that the snake is moving away from us. So, who's more afraid here?
The speaker (or us, as the poems draws us in), or the snake?
Rhythm note: By now, and particularly if you're reading out loud, you should notice a kind of regular rhythm to these stanzas. It's called a "ballad" meter, about which we say much, much more over in "Form and Meter."