Hookworms are very small worms. They're usually 1–2 cm (approx. 0.4–0.8 in) in length and appear as a thin string. They often have a hooked end. Even if your pet has hookworms, you may not see them in the feces. They're usually attached to your pet's intestinal walls and release eggs into the feces, which are visible under a microscope.Hookworms are very small worms. They're usually 1–2 cm (approx. 0.4–0.8 in) in length and appear as a thin string. They often have a hooked end. Even if your pet has hookworms, you may not see them in the feces. They're usually attached to your pet's intestinal walls and release eggs into the feces, which are visible under a microscope.