Echinoderms, a phylum of marine invertebrates including sea stars, sea urchins, and sea cucumbers, possess a unique physiology unlike most animals. They do not have blood or a pumping heart in the way humans or other vertebrates do. Instead of a centralized, closed circulatory system, echinoderms rely on specialized fluid-filled body cavities and a hydraulic network to manage internal transport. This arrangement allows them to function efficiently without a traditional vascular system.
Why Echinoderms Lack a True Circulatory System
A true circulatory system requires a centralized pump, such as a heart, major blood vessels, and specialized respiratory pigments for oxygen transport. Echinoderms lack a true heart to drive high-pressure fluid circulation. Their internal transport is managed by a series of open, reduced, fluid-filled spaces.
The fluid moving through these spaces often lacks specialized respiratory pigment, such as hemoglobin or hemocyanin, causing it to appear nearly colorless. This simplified internal architecture is enabled by the echinoderm’s distinctive pentaradial body plan and low metabolic rate.
Their slow-moving, benthic lifestyle does not demand the rapid, high-volume oxygen delivery provided by a complex, closed circulatory system. The echinoderm body structure, with its radial symmetry, maximizes the surface area of internal tissues close to the external environment. This proximity simplifies gas exchange and nutrient distribution.
The Water Vascular System and Its Function
The most distinctive feature of echinoderms is their hydraulic network, known as the water vascular system (WVS). This unique network of fluid-filled canals is derived from one of their coelomic cavities and is primarily engineered for mechanical functions. The WVS is used for locomotion, food manipulation, and adherence to surfaces.
Water enters the system through the madreporite, a sieve-like plate on the aboral surface, and flows into an internal stone canal. This canal connects to a ring canal encircling the mouth, from which five radial canals extend into each arm. Along the radial canals are hundreds of small, muscular sacs called ampullae, which connect to the external tube feet (podia).
The contraction of the ampullae forces fluid into the tube feet, causing them to extend and creating the hydraulic pressure necessary for movement and powerful suction. While the WVS is fluid-filled, its main purpose is to power movement and feeding, not to serve as the primary physiological transport highway for oxygen and nutrients.
Internal Transport of Nutrients and Gases
Internal transport is handled by two main mechanisms: the coelomic fluid and the hemal system. Coelomic fluid is a watery solution filling the vast coelomic cavities surrounding the internal organs. This fluid distributes digested nutrients, transports metabolic waste, and carries oxygen throughout the body tissues.
Suspended within this fluid are specialized cells known as coelomocytes, which perform functions analogous to blood cells in other animals. These cells actively engulf waste particles and bacteria, aiding in immune defense and the removal of cellular debris. The coelomic fluid is circulated by the movement of cilia, tiny hair-like projections that line the internal cavities, creating gentle currents for distribution.
Echinoderms also possess channels known as the hemal or perihemal system. This network is involved in distributing nutrients absorbed from the digestive system. Gas exchange occurs directly between the seawater and the coelomic fluid through thin body surface structures. Oxygen diffuses across the delicate membranes of the tube feet, dermal branchiae (skin gills), or the specialized respiratory trees found in sea cucumbers. The oxygen dissolves into the circulating coelomic fluid, and carbon dioxide diffuses back out through these surfaces.