Body Structure & Physiology





Water Vascular System






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Web Design By:

Kaili Jackson

University of Alaska Southeast Juneau, AK



General internal anatomy of a cucumarid sea cucumber. Picture from Lambert (1997).

The body wall of the sea cucumber lacks the rigidity found in other echinoderms because the calcareous plates (ossicles) that compose the skeletal system are very small and widely isolated. These ossicles are secreted by special cells called sclerocytes and are embedded in the outer layers of the skin. Ossicles are species-specific in structure and complexity, and can be used to identify species.  

Ossicles can function to protect juveniles, as the ossicles are oversized and armor-like in the smaller cucumbers. They probably do not retain any protective qualities in the adults.
Due to the lack of hard parts in the body of the Holothuroideans, their fossil record is hard to follow, but we do know that the first evidence of sea cucumbers dates back about 400 million years ago. Only twelve complete fossil species have been collected.

Skin ossicles of cucumaria vegae. Scale is 100 micrometers. Picture from Lambert (1997).

The nervous system is decentralized (meaning there is no brain) and made up of a nerve ring, radial nerves, and their branches.  There is also a nerve net located within the skin that can detect touch and chemical stimuli.

With no heart, the blood system cannot be considered a complete circulatory system. Most of the blood “vessels” are discrete body wall spaces and blood cells found within the coelom. By contracting the body wall muscles, a form of circulation takes place as the coelom pushes fluid back and forth.

Sea cucumbers “breathe” via hollow, branched, Y-shaped organs called respiratory trees, which lie within the body cavity alongside the intestine. A muscular cavity, called the cloaca, forms the base of the respiratory tree. The cucumbers will “breathe” by expanding the cloaca, which draws oxygenated water into the anus. This is what the term “anal breathing” refers to. Circular muscles, called sphincters, will close the cloaca and force the water into the respiratory trees, where oxygen will be transferred across a thin membrane into the body cavity fluids.

One interesting characteristic of sea cucumbers is the ability to go from completely flaccid to incredibly rigid at will. This was studied in Cucumaria frondosa, and it was determined that this process is calcium dependent.

The respiratory tree of a dissected sea cucumber in water. Photo taken by Kaili Jackson.
A very important physiological characteristic of all echinoderms is the water vascular system.