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Bilateral Symmetry and Cephalization

Bilateral Symmetry and Cephalization - sexually mature...

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Bilateral Symmetry and Cephalization: Phylum Platyhelminthes Parasitic members of this phylum, such as flukes and tapeworms, are characterized by these modifications: 1. loss of cephalization producing a head bearing hooks and suckers to attach to the host as opposed to the sensory organs of free-living forms 2. extensive development of the reproductive system coinciding with the loss of other systems (what do they do but gain food from the host's digestion and reproduce, anyway?) 3. lack of a well-developed nervous and gastrovascular system (the live in a fairly stable environment and the host has already digested their food) 4. development of a tegument that protects them from host digestive juices Both flukes and tapeworms use secondary or intermediate hosts to transport the species from primary host to primary host. The primary host is infected with the
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Unformatted text preview: sexually mature adult while the secondary host contains the larval stage(s). The Phylum Nemertea: Ribbon Worms The phylum Nemertea include approximately 650 species of marine ribbon worms. Ribbon worms have a distinctive eversible proboscis stored in a rhynchocoel. When the walls of the rhynchocoel contract, the proboscis extends out of the body. The proboscis is a long, hollow tube that can be everted and shot outward through a pore located just above the mouth. It is used primarily for prey capture, and for defense, locomotion, and burrowing. This phylum is included as the organisms are also triploblastic. Several of the fossils from the Cambrian-aged Burgess Shale are interpreted as ribbon worms, and some extremely long worms have been found in the Mediterranean (up to 100 feet long) and in the ocean under the Antarctic ice shelves....
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