People have lots of ways of getting around: cars, buses, bikes, motorcycles, boats, subways, trains, airplanes. Plus, there's always the old-fashioned option of walking. Even if you break a leg, you can still get around on crutches. But they don't make crutches for turtles. And for a turtle, survival depends on mobility. "Turtle" is chock-full of imageryand figurative language relating to movement. This turtle may be slow, but she's always "on her way" somewhere, either to find food or to avoid becoming food for another animal. And every journey is risky business.
Line 2: The idea of mobility pops up immediately in the form of a comical boating metaphor, as the speaker describes the turtle as "a four-oared helmet." In an equally humorous metaphor, the turtle is compared to a "barely mobile hard roll," suggesting the image of a roll rolling along like a little car. Underlying these amusing metaphors is the serious issue of survival: a roll is a form of food, and a helmet is a form of protection.
Line 4: The speaker extends the metaphor of the four-oared helmet with a reference to "rowing." For the turtle, this voyage is a business trip, not a vacation cruise. She must travel to find "the grasses that she eats."
Lines 5-6: For a turtle, everything depends on movement, but every movement is heavy and cumbersome, "like dragging / A packing-case places." This simile emphasizes the difficulty of the turtle's struggle to move and to survive.
Lines 8-9: If movement is necessary to survival, then an inability to move is cause for panic (see "Freedom and Confinement" in the "Themes" section). If a turtle is "stuck up to her axle" (another automotive metaphor), you better believe she's whipping out her cell phone to call AAA and 911 ASAP!
Lines 10-11: Ditto for falling into a ditch. Have you ever seen a turtle on its back, helplessly waving its legs, betrayed by the same heavy shell that normally serves as such a cozy and secure roof? This situation is even more perilous than being stuck in the mud, as a turtle on its back is totally exposed to predators.
Lines 12-13: If you were a turtle, wouldn't you dream of flying? And if you were a wealthy turtle, wouldn't you definitely fly first-class, maybe even charter a private jet? It would be such a relief to check your baggage instead of dragging a heavy shell. The image of "wings" in line 13 is especially poignant because this turtle is so beaten down (or so humble—see "Humility" in the "Themes" section) that she can't even imagine flying.