Crinoids, Star Fish, Brittle Stars, Sea Cumbers et al


Echinodermata is a phylum composed exclusively of marine invertebrates (i.e. there are none on land or in fresh water). The fossil record of Echinodermata extends back to the Precambrian.

Echinoderms mean "spiny skin" in Greek. Most, although not all, echinoderms have spiny skins. Most also have five arms or rays (appendages), but there are some exceptions. Echinoderms are radially symmetrical animals, which means that the main body is the hub with tentacles, like spokes of a wheel radiating out from this body hub, much like a bicycle wheel. If you think of a star fish this is easy to visualize. The echinoderm larvae is bilaterally symmetrical (i.e. two parts), but as they mature they become radially symmetrical (usually with five parts, arms, appendages or parts--again, like a star fish).

Echinoderms include the fossil Crinoids and modern Crinoids (called Sea Lilies and Feather Stars), as well as Brittle Stars, Sea Cumbers and other animals. There are over 6,000 species of echinoderms.

Radial symmetry means that the body is a hub, like a bicycle wheel, and tentacles are spokes coming out of it (think of a starfish). As larvae, echinoderms are bilaterally symmetrical. As they mature, they become radially symmetrical.

The Echinoderm clad is supported by certain unique features:

  1. a water vascular system;
  2. a system of endoskeletal elements (arising from mesoderm) composed of a calcareous meshwork called stereom;
  3. basic 5-part symmetry (often modified later in ontogeny) expressed in the development of 5 primary lobes in the hydrocoel of the rudiment;
  4. construction of ambulacra by the addition of new plates adjacent to the terminal (or ocular) plate that marks the ends of the radial canals; and
  5. a diffuse sub-epithelial nervous system.

Echinoderms are made up of interlocking blocks. They have a single main entrance and numerous slits like windows. They could be compared with a castle, with the main entrance and the slits like windows for air and a place to mount a defense. The skeleton is made up of CaCO³, i.e. Calcium carbonate, plates and spines. This skeleton is then covered with a skin, or epidermis. This means Echinoderms have an endoskelton, i.e. an inside skeleton.

The size and the way the plates fit varies among the different type of echinoderms. In sea urchins, the plates are tightly fitted together whereas in a starfish, the plates are more loosely fitted together. In a sea cucumber the plates are usually just microscopic in size.

The plates of echinoderms are intricate with a minute microstructure. Using an electron microscope reveals that the echinoderm skeletal structure is not solid blocks, but the CaCO³ forming a structure called a stereom. Each of the skeletal parts of an echinoderm is a single crystal of CaCO³, which is structured and branched in extremely fine detail.

An echinoderm moves about, defends itself and breaths through some special structures which protrude out between the skeletal plates. Typically, these are tube feet, pedicellaria, and gills. A water-vascular system is present in all echinoderms. These are water filled canals branching out from a ring canal which encircles the echinoderm's gut.

These canals run out to podia, or tube feet. Tube feet are sucker-like appendages which echinoderms use to move about, grip the substrate, or allow them to move or manipulate objects. Echinoderms are able to extend and retract these tube feet. In doing so echinoderms use create hydraulic pressure within their water-vascular system. To prevent small organisms from alighting and settlement on an echinoderm's body, it has Pedicellaria. These are small skeletal elements which are snapper-like.

An echinoderm has a coelom, which is an open body cavity lined with tissue and filled with fluid. They have large gonads (i.e. sex organs) and most have a complete gut or alimentary canal. However, there are some strange things about echinoderms.

Sea urchins scrap algae from rocks with a structure, created of five large teeth arranged into what is called "Aristotle's lantern". Many starfish turn their stomachs inside out, through their mouth, to feed.

Echinoderms (except holothurians) do not have a respiratory system and only the most elementary circulatory system. The water-vascular system, mentioned above, operates to provide some of these systems found in other animals. Echinoderms generally have poorly developed nervous and sensory systems.

This page is still under construction, so more material will be added later.

THANKS TO THE FOLLOWING SOURCES of information used on this page:

CAS Echinoderm Website http://www.calacademy.org/research/izg/echinoderm/.
Ocean Animals from Missouri Botanical Garden http://mbgnet.mobot.org/salt/animals/echinod.htm
University of California @ Berkeley website http://www.ucmp.berkeley.edu/echinodermata/echinomm.html