Egg capsules, also known as ovisacs, are a type of reproductive structure found in some species of invertebrates. They are typically composed of a tough outer shell that protects the eggs until they are ready to hatch. The egg capsules are usually laid in clusters and can contain anywhere from one to several hundred eggs. The eggs are often fertilized internally by the female before being released into the environment. Egg capsules provide protection for the developing embryos and can help ensure their survival until they hatch.