Papilla (worms)

For dermal papillae in humans, see Dermis § Dermal_papillae.

Sensory papillae covering the exterior of a velvet worm

Papillae (sg.: papilla) are small, protruding structures found on the surface of certain worms, specifically polychaetes and onychophorans.

In polychaetes, these papillae are tiny, fleshy projections on the worm's body or parapodia, often associated with sensory and locomotive functions. Some papillae have sensory receptors to help the worm detect environmental changes, such as touch, water currents, or chemical signals, and support movement by working with parapodia to aid grip and friction during crawling or burrowing. However, papillae on the bodies of bristle worms play a protective role, specifically in the secretion of mucus used to deter predators.

In onychophorans, papillae are microscopic or macroscopic projections scattered across their soft, velvety skin, giving the worms their distinctive texture and appearance. Many papillae house glandular openings for producing mucus or other secretions used for capturing prey. They also help to maintain the worm's moisture balance by reducing water loss, as velvet worms are highly sensitive to desiccation.