tegumentum

Tegumentum is a Latin term meaning covering or wrapper. In modern biology, tegument refers to the outer protective layer or integument of an organism, or to the outer layer of certain cells or tissues. The term is used in Latinized scientific contexts to describe the body’s external covering, distinct from internal tissues, and is often applied when describing organisms with an atypical or specialized epidermis.

In invertebrates, particularly some flatworms (Platyhelminthes), the tegument is a syncytial outer layer that covers the body and forms the interface with the environment. It may be integrated with ciliated cells or microtrichs and is adapted for functions such as absorption, secretion, osmoregulation, and protection.

In parasitology, the tegument is a key structural feature of many parasitic helminths, including cestodes (tapeworms) and trematodes (flukes), and some larval nematodes. The tegument is typically a syncytial exterior that lacks distinct cell boundaries, acting as a selective barrier against host defenses while facilitating nutrient uptake and waste excretion. It is often enriched with surface projections like microtriches to increase absorptive area.

In vertebrates, the usual term for the outer covering is epidermis or integument; tegument is used less frequently and mainly in historical, comparative, or broad descriptive contexts to refer to a body’s external covering.

See also: integument, epidermis, syncytium.