Rhabdus

Rhabdus (plural: rhabdi) are microscopic, rod-shaped structures found within the epidermal cells of certain turbellarian flatworms, particularly those belonging to the class Rhabditophora. These structures are ejected from the epidermal cells when the worm is disturbed or attacked, forming a protective mucous sheath around the animal.

Rhabdi are composed primarily of protein and are thought to be formed in specialized cells called rhabditogenous glands or rhabdite-forming cells located beneath the epidermis. Once formed, they migrate to the epidermal cells and are stored until needed.

The exact composition of rhabdi can vary between different species of turbellarians. Upon ejection, the rhabdi typically dissolve in water, creating a gelatinous or mucous-like layer that can deter predators or provide a temporary escape mechanism for the worm. The sheath can also protect the worm from desiccation in terrestrial or semi-aquatic environments.

The presence and structure of rhabdi are important characteristics used in the classification and identification of turbellarian flatworms. The morphology and distribution of rhabdi can be species-specific, providing valuable taxonomic information.