How do protective eggshells differ between aquatic and terrestrial flatworms?

The composition of protective eggshells in flatworms varies based on the environmental demands of the species, specifically regarding their reliance on aquatic or terrestrial habitats.1

• Aquatic Flatworms: For species that live primarily in water or possess an aqueous phase in their life cycle, the eggshell is typically composed of a hardened protein known as sclerotin, or tanned protein.1
• Terrestrial Flatworms: Species that are primarily terrestrial or have a terrestrial phase in their life cycle utilize keratin to form their eggshells. Keratin is a tougher material than sclerotin, providing the embryo with greater resistance to adverse environmental conditions.1

In both groups, the components for these shells are produced by the vitellaria (yolk glands), which are specialized structures within the female reproductive system. Additionally, some freshwater planarians further adapt to seasonal changes by producing thin-shelled summer eggs for rapid development and thick-shelled winter eggs that are resistant to freezing.12