Asārīʿ refers to small, elongated, soft creatures found in sand. Ẓaby is the locality associated with them. Masāwīk is the plural of miswāk, and isḥil denotes a tree or shrub whose slender twigs can serve as tooth-cleaning sticks.
The poet singles out the fingers' slender, smooth, elongated shape. He therefore selects one image from the animal world and another from the plant world, each making that form tangible.
Aw introduces alternative similes, not a claim that the two objects are identical. Sound analysis should keep both comparators and their separate sensory associations visible.