incurrent

Incurrent is a term used mainly in biology to describe the direction of a fluid flow into an organism, chamber, or structure. It refers to inward movement of a medium, such as water or air, as opposed to excurrent, which denotes outward flow. The word is commonly applied in the study of aquatic invertebrates and their feeding and respiratory systems, where directed water flow is essential for filtering, gas exchange, or waste removal.

Etymology-wise, incurrent derives from the Latin in- meaning “into” or “toward” and current, meaning flow or running. In scientific usage, the distinction between incurrent and excurrent helps describe the architecture and function of an organism’s aquiferous, respiratory, or excretory pathways.

In practical examples, sponges (phylum Porifera) use incurrent canals and ostia to draw water into their bodies, where choanocytes filter food particles before the water exits through excurrent openings. Bivalve mollusks often possess incurrent siphons that draw in seawater for respiration and feeding, while water exits via excurrent siphons. In other contexts, incurrent flow may refer to any inward-directed fluid movement within a system, including engineered or experimental setups.

Incurrent is rarely used outside of biology, but the concept of inward versus outward flow is a common framing in physiology, anatomy, and fluid dynamics. See also excurrent, ostia, siphon, and aquiferous system.