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Sea cucumber

Why the sea cucumber is classified as an echinoderm

Cellular mechanisms of intestine regeneration in the sea cucumber, Holothuria glaberrimaSelenka (Holothuroidea:Echinodermata)

JE García‐Arrarás

Skeletal homologies of echinoderms

Mooj,R

Phylogeny

Holothuroidea (sea cucumbers) are one of five classes that make up the phylum Echinodermata. Echinodermata is known as one of the most distinctive and diverse phyla ranging from starfish to urchins to sea cucumbers and many other organisms. The echinoderms are mainly distinguished from other phyla by their body plan and organization. While the organisms in this phylum may not all look the same from the outside, their make-up is another story.

There are three main characteristics that all echinoderms share. When mature, echinoderms have a pentamerous radial symmetry. While this can easily be seen in a sea star or brittle star, in the sea cucumber it is less distinct and seen in their five primary tentacles. The pentamerous radial symmetry can also be seen in their five ambulacral canals (Smirnov, 2013). The ambulacral canals are used in their water vascular system which is another characteristic that binds this phylum together. T

The water vascular system that develops from their middle coelom or hydrocoel. Echinoderms use this system for many things including movement by pushing water in and out of their podia or "tube feet". Echinoderms tube feet (including sea cucumbers) can be seen aligned along the side of their axes.

While echinoderms are invertebrates, meaning they do not have a spine, they do all have an endoskeleton that is secreted by the mesenchyme. This endoskeleton is composed of plates called ossicles. They are always internal but may only be covered by a thin epidermal layer like in sea urchin's spines. In the sea cucumber the ossicles are only found in the dermis, making them a very supple organism. For most echinoderms, their ossicles are found in units making up a three dimensional structure. However, in sea cucumbers the ossicles are found in a two-dimensional network (Florkin, 1969).

All echinoderms also posses a trait called mutable collagenous tissues.

The majority of the organisms on our planet falling under the Kindom Animalia are invertebrates. There are many phyla that fall underneath the inverts, but the one most closely related to us, vertebrates, is the phylum Echinodermata. The Echinoderms are deuterostomes unlike most other invert phyla. Homo Sapiens are also deuterostomes, which is a key part of development. Echinodermata has several characteristics that set them apart from the rest of their phylum and closer to the vertebrates. One of these characteristics is there mutable collagenous tissue or MCT. MCT is a fascinating mechanism where the tissues can change their mechanical properties in a very short amount of time. Tissues can go from soft to stiff all under the control of the nervous system. Every class under the phylum Echinodermata has this MCT but uses it in different ways. The Asteroids, sea stars, can detach limbs for self defense and then regenerate them. The Crinoidea, sea fans, can go from stiff to limp depending on the current for optimal filter feeding. The Echinoidea, sand dollars, use MCT to grow and replace their rows of teeth when they need new ones. The Holothuroidea, sea cucumbers, use MCT to eviscerate their gut as a self-defense response. MCT can be used in many ways but the cellular level and mechanics function the same throughout. One common trend in the uses of MCT is that it is generally used for self defense mechanisms and in regeneration.

Wilke, I. 2005. Mutable Collagenous Tissue: Overview and Biotechnological Perspective. Progress in Molecular and Subcellular Biology Subseries Marine Molecular Biotechnology, Echinodermata, 221-249.

Florkin, M., Scheer, B. (1969) Chemical Zoology, Echinodermata, nematoda, and acanthocephala. general characteristics. volume 3. 3-16.

Smirnov, A. (2013). Sea cucumbers symmetry (echinodermata:holothurodia) Zoological Institute, Russian Academy of Sciences. 48, 1215-1236

Paleontological Journal, 2014, Vol. 48, No. 12, pp. 1215–1236. © Pleiades Publishing, Ltd., 2014. (pgs 1217, 1218)

= Echinoderms Through Time = By Bruno David, Alain Guille, Jean-Pierre Fera