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Download 30.4 The Ecdysozoa
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30.4 The Ecdysozoa The ecdysozoans are protostomes, as are the trochozoa. The term ecdysis means molting, and both roundworms and arthropods, which belong to this group, periodically shed their outer covering. Roundworms Roundworms are nonsegmented worms that are prevalent in almost any environment. They possess a pseudocoelom. Ascaris A female Ascaris is very reproductively prolific. They are most commonly parasites of humans and pigs. Other Roundworms Trichinosis is a fairly serious infection cause by eating pork that is not fully cooked that contains encysted larvae of the round worm Trichinella spiralis. Elephantiasis is caused by a roundworm called the filarial worm. Pinworm and hookworm infections are more common in the United States. Arthropods Arthropods are extremely diverse. Over one million species have been discovered and described. Arthropods have jointed appendages and an exoskeleton made of chitin. They are segmented and have a well-developed nervous system that includes a brain and a ventral solid nerve cord. They have a variety of respiratory organs and have reduced competition through metamorphosis. Crustaceans Crustaceans include barnacles, shrimps, lobsters, and crabs. They are named for their hard shells. Internal Organs In the crayfish, the digestive system includes a stomach. The coelom is reduced to a space around the reproductive system. They have an open circulatory system. The nervous system is very similar to that of an earthworm. Insects Insects are very numerous and diverse. Insects have a body that is divided into a head, a thorax, and an abdomen. The head bears sensory antennae, a pair of compound eyes, and several simple eyes. The mouthparts are adapted for the type of food. Wings may be present. Internal Organs In the grasshopper, there is an excretory system, a respiratory system, and a circulatory system. Reproduction and Development Metamorphosis is a change in form and physiology that occurs as an immature stage, called a larva, becomes an adult. Comparison with Crayfish A comparison of a grasshopper with a crayfish reveals the adaptations to a terrestrial environment. Arachnids The arachnids include scorpions, spiders, ticks, and mites. They have six pairs of appendages.
