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Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html) 11/15/99 Text (7th ed.): Ch. 25, pp. 542-557

possible web site: http://www.ucmp.berkeley.edu/echinodermata/echinodermata.html

1. Phylum Echinodermata ("spiny skin"): A marine, bottom-dwelling phylum of around 6000 species, the echinoderms include such familiar organisms as sea stars and sea urchins. Major features include: pentaradial symmetry (parts arranged in fives) around a central mouth, an internal skeleton of calcium carbonate plates often with protruding spines, and water-vascular system with protruding tube feet. There is no distinct head or brain, and most organ systems have a circumoral (mouth-surrounding) ring element and five radial elements.

2. The dermal endoskeleton. It underlies the skin in the dermis and consists of a lattice or network of ossicles, which can be separate or in the form of large radiating plates sutured together to form a shell. Both ossicles and plates can contain knobs or spines.

3. The water-vascular system. This is actually a subdivision of the coelom or body cavity, making up peritoneal-lined tubules. Originally used for food gathering in sea lily-like fossil echinoderms, it is now also used for movement in forms such as sea stars.2 The water-vascular system is filled with seawater, and can assist with respiration and excretion by involving so much surface area. A sieve plate or madreporite takes the water in, which is then transported through a stone canal (cartilage enforced) to the ring canal around the mouth. There are then radial canals extending out to the arms or rays. In the arms, lateral canals are given off of the radial canal to reach the ampulla and podium (tube foot). The ampulla works like an old-fashioned bicycle horn; when this bulb contracts, the tube foot protrudes. As the ampulla relaxes, the tube foot shortens, and a suction disk at the base adheres to the surface. Tube feet line ambulacral grooves.

4. This phylum design of perforated dermal endoskeleton limits the group to only a few designs and life styles. Water-vascular system and tube feet, unique to echinoderms, could only exist with the support of the endoskeleton.3 The ossicles of the endoskeleton form units that end up enlarging in geometrical patterns during growth of the organism. Presumably there was selective pressure for a bottom-dwelling lifestyle, where the radial nature can allow the echinoderm to respond in any direction to food, changing conditions, etc. An explanation for the pentaradial symmetry could be the avoidance of"weak spots" by not having joints opposite each other.2

5. Major classes of echinoderms:

a. Class Asteroidea ("star form"): sea stars. Five arms blend into central disc. Tough but flexible skins since ossicles separately embedded. Many are serious predators of oysters, etc. as they open the valves of a clam and evert their (cardiac) stomachs into the opening.

b. Class Echinoidea ("spiny form"): sea urchins and sand dollars. Ossicles form a continuous fused armor in the form of plates. Long protruding spines are protective in sea urchins and short spines used for burrowing in sand dollars.

c. Class Ophiuroidea ("snake tail form"): brittle or serpent stars. Mobile flexible arms clearly demarcated from central disc. Dermal ossicles form a continuously articulated armor, especially along the arms. Tend to be secretive small particle feeders, in contrast to sea stars.

d. Class Holothuroidea ("sea cucumber form"): sea cucumbers. Central body is elongated. Creeps along bottom and/or burrows in sand, using tentacles (large modified tube feet) around mouth to rake in sediment.

e. Class Crinoidea ("lily form"): sea lilies and feather stars. Stalked, mouth directed upwards, food collected by tube feet in the branched arms. Ossicles almost continuous but articulated. Reduced central disc. Many fossil forms.

As surprising at it might seem, the echinoderms are the closest relative to our phylum, Phylum Chordata, of the organisms that we have examined this semester. Some unusual, bilaterally symmetrical, echinoderm fossil groups seem to share some chordate (our phylum) features, such as gill slits.1 Also, echinoderms have an early pattern of development in their early stages that is similar to ours (see below). During metamorphosis to adulthood, though, most echinoderm larvae will form a new mouth off to the left side and radially symmetrical organs will develop around it.1

6. Aspects of animal development; useful terms to know.

a. zygote: a fertilized egg (union of male and female sex cells)

b. blastomere: cells that are derived from the zygote by cell division

c. blastula: hollow ball formed by the dividing blastomeres

d. gastrula: two layered embryo, often formed from blastula by invagination or gastrulation

e. archenteron: the new cavity in the gastrula formed by the invagination

f. blastopore: the new opening to the archenteron where the invagination occurred

g. ectoderm, endoderm, mesoderm: primary germ layers discussed previously


The typical developmental pathway of an animal is to have an egg, a zygote, a blastula, a gastrula, a larva, and then an adult. There are two major ways of going about development:

7. Protostome development: typical of most invertebrates. Zygote formation, divisions into blastomeres. These blastomeres tend to be determinate, that is, their developmental fate is fixed from early on. These blastomeres in many protostome groups undergo spiral cleavage to form the blastula. The gastrula is formed by invagination, which creates the germ layers of ectoderm and endoderm, as well as the new opening of the blastopore and the new cavity of the archenteron. The blastopore forms the future mouth and the archenteron forms the future digestive tract. Hence,"protostome," or the first opening (blastopore) being the mouth. Primitive mesoderm cells are created and lie between the endoderm and ectoderm.

8. Deuterostome development: seen in the echinoderms and chordates (phylum that includes vertebrates), thus helping to link them. Zygote formation, divisions into blastomeres. These blastomeres tend to be indeterminate, that is, their developmental fate is not fixed early on. The blastomeres tend to undergo radial cleavage at 90o. Gastrulation occurs, and the now-formed archenteron again will be the future digestive tract. The blastopore becomes the future anus, and the future mouth will develop at the other end, hence"deuterostome."


1. CP Hickman et al., Biology of Animals, 7th ed. (WCB McGraw-Hill, Boston, 1998), pp. 323-324, 548, 555, 570.

2. SA Miller, JP Harley, Zoology, 2nd ed. (WC Brown, Dubuque, IA, 1994), pp. 389, 390, 398

3. WD Russell-Hunter, A Life of Invertebrates (Macmillan, New York, 1979), p. 481-487, 505-506.


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