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Anatomy of Animals


The echinoderms include only marine animals -- starfishes, sea urchins, sea cucumbers, and sand dollars (see Figure 17). The starfish will be studied as the representative for this group. Their unique feature is the water vascular system, which is used as a means of locomotion. They also have a carbonaceous endoskeleton, whose projecting spines give the phylum its name - "spiny skin" in Greek. The echinoderms seem the most unpromising of all as potential ancestors of the vertebrates.
Echinoderms They are radially symmetrical, in contrast to vertebrates; they have no internal skeleton, no trace of any of the three major chordate characters of notochord, nerve cord, or gill slits, and they have many peculiar and complicated organs of their own. But the embryology sheds an unexpected gleam of light. The early embryo of the echinoderm is a tiny creature, which floats freely in the sea water. Unlike the adult, the larva is bilaterally symmetrical, suggesting that the radial symmetry of the starfish is a secondary affair, assumed when the ancestors of these forms look up a sedentary existence. Then, too, the type of development of certain of the body cavities is identical with that found in the embryos of some primitive vertebrates. It is believed that the bilateral larva developed types which retained the original symmetry, and gradually evolved into the chordates and, finally, the true vertebrates.

Figure 17 Echinoderms
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