Cocoons of the leech, Theromyzon tessulatum, are secreted underwater and sealed by two opercula (plugs) that are asymmetrically positioned on the upper aspect of the cocoon membrane. The opercula are protein-based, but their amino acid composition appears to differ from the surrounding cocoon membrane. Fluid deposited inside the cocoon by the parent leech contains several proteins ranging in size from ∼40–100 kDa, most of which have pI's below 6.8. While inside the cocoon, late stage embryos display peristaltic contractions that may contribute to weakening of the opercula/membrane boundary. Juveniles break through the opercula after 2–3 wk and exit the cocoon over the course of several hours. The asymmetrical position of the opercula on the upper surface of the cocoon membrane may facilitate contact between the brooding parent leech and the emerging young, which attach to the venter of the parent and are brought to their first blood meal.