spinules

Spinule is a general term used in biology to denote a small spine or spine-like projection on a surface. The word derives from Latin spinula, a diminutive of spina (thorn). Spinules are found in a variety of organisms and tissues, and they range in size from microscopic to several millimeters, but they are typically finer and shorter than true spines.

In flatworms (phylum Platyhelminthes), spinules are structures associated with the tegument—the syncytial outer covering of many parasitic species such as cestodes and some trematodes. Tegumental spinules are minute projections that increase the apparent surface area and can influence interactions with the host tissues. In this group, spinules and related projections are often described as part of species-level taxonomic characters used to distinguish closely related parasites.

In arthropods, spinules occur as small cuticular projections on the exoskeleton. They may be located near setae or along sensory or joint regions. The exact function of these spinules can vary, contributing to surface texture, friction, protection, or the functioning of sensory structures. In many cases they aid the organism in adapting to its physical environment or specific ecological niche.

Spinules are distinguished from larger structures such as spines by their small size and morphology, though terminology can vary across taxa and literature. They are typically studied in the contexts of anatomy, morphology, and taxonomy, where their presence, arrangement, and size can provide diagnostic information about species or functional adaptations.