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• The two major evolutionary lines of coelomate animals-the protostomes and the deuterostomes-are both represented among the oldest known fossils of multicellular animals, dating back some 650 million years.
• In the protostomes, the mouth develops from or near the blastopore, and the early divisions of the embryo are spiral. At early stages of development, the fate of the individual cells is already determined, and they cannot develop individually into a whole animal.
• In the deuterostomes, the anus develops from or near the blastopore, and the mouth forms subsequently on another part of the gastrula. The early divisions of the embryo are radial. At early stages of development, each cell of the embryo can differentiate into a whole animal.
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1. What patterns of embryonic development related to cleavage and the blastopore occur in protostome coelomates? What patterns occur in deuterostome coelomates?
2. Which major coelomate phyla are protostomes and which are deuterostomes? How does the early developmental fate of cells differ between the two groups? How is the development of the coelom from mesodermal tissue different between them?
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• Echinoderms are exclusively marine deuterostomes that are radially symmetrical as adults.
• The epidermis of an echinoderm stretches over an endoskeleton made of separate or fused calcium-rich plates.
• Echinoderms use a unique water-vascular system that
includes tube feet for locomotion and feeding.
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3. What type of symmetry and body plan do adult echinoderms exhibit?
4. What is the composition and location of the echinoderm skeleton?
5. How do echinoderms respire? How developed is their digestive system?
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• Crinoids are sessile for some or all of their lives and have a mouth and anus located on the upper surface of the animal.
• Sea stars are active predators that move about on their tube feet.
• Brittle stars use their tube feet for feeding and move about using two arms at a time.
• The endoskeletons of sea urchins and sand dollars consist of fused calcareous plates that have been well preserved in the fossil record.
• The endoskeletons of sea cucumbers are drastically reduced and separated, making them soft-bodied.
• Sea daisies are a newly described class of echinoderms
with disc-shaped bodies.
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6. In what two ways do members of the phylum Echinodermata reproduce? What type of larva do they possess?
7. How do sea cucumbers superficially differ from other echinoderms? How are some of their tube feet specially modified? What is the extent of their skeleton? What is the function of their unique respiratory tree? How is their reproduction different from that of other echinoderms?
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