Polian vesicle

zoology

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

July 1, 2026

Quick Summary

The term "Polian vesicle" refers to bulbous cavities found within the water-vascular system of certain echinoderms, such as starfish and sea cucumbers. This internal hydraulic system is crucial for echinoderm movement, primarily through the extension and retraction of tube feet. Polian vesicles specifically function to maintain pressure within this system and store reserve fluid. While most echinoderms have these structures, crinoids (sea lilies) are an exception, lacking them entirely. The water-vascular system is a defining characteristic of echinoderms, with evidence suggesting its presence even in ancient fossil forms.1

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

3 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

echinoderm
Water-vascular system
…holothurians has bulbous cavities called Polian vesicles, which apparently maintain pressure in the system and hold reserve supplies of fluid; ophiuroids have four or more vesicles, asteroids five, holothurians from one to 50. Crinoids lack Polian vesicles, and echinoids have five structures known as either Polian vesicles or spongy bodies.
Read More
…holothurians has bulbous cavities called Polian vesicles, which apparently maintain pressure in the system and hold reserve supplies of fluid; ophiuroids have four or more vesicles, asteroids five, holothurians from one to 50. Crinoids lack Polian vesicles, and echinoids have five structures known as either Polian vesicles or spongy bodies.
Read More