acicula

Acicula, plural aciculae (also aciculi in some usages), is a term used in invertebrate anatomy to describe a small, needle‑like internal support rod found in the parapodia of many polychaete annelids. Each parapodium can bear one or more aciculi, which are typically embedded in the parapodial wall and may protrude dorsally or into the lobus. They are usually composed of chitin or calcium carbonate and can be calcified in certain species.

The primary function of the acicula is to act as an internal spine that stiffens and supports the parapodium, aiding locomotion by providing a rigid axis against which the parapodial muscles can work. This support helps the worm anchor its appendages and push against the substrate when crawling or burrowing. The morphology and degree of calcification of aciculi vary among taxa and are often considered in taxonomic descriptions of polychaetes.

In comparative context, aciculae are distinct from the external chaetae or bristles that polychaetes use for locomotion and interaction with the environment; aciculae provide internal structural support rather than propulsion. The presence, absence, and specific characteristics of aciculae can thus be useful characters in polychaete systematics and evolution.

Etymology and scope: the term derives from Latin acicula, a diminutive of acus meaning "needle." While most commonly applied to parapodial rods in polychaetes, the word has appeared in other contexts to describe similarly needle‑like structures in invertebrates and in fossil descriptions, though usage in those contexts is less standardized.