BIO 203 Lecture Notes - Lecture 36: Flatworm, The Fluid, Trochophore
9 Dec 2016
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Document Summary
Lophotrochozoa possess either a lophophore or trochophore larva. Ecdysozoa have to mold their outer cuticle or skeleton to grow. Flatworms vary from a millimeter to many meters in length. Internal regions are filled with mesoderm and a spongy mass of parenchyma. It is caused by the cercaria of trematodes penetrating the skin but not finding an appropriate host they are unable to transform and die. Does evolution always result in increased complexity: no, examples: Body cavities (coela) have evolved > 10 times in the bilateria. A coelom provides more space for organs and surface area for exchange allowing for greater size and complexity of organisms. True metamerism is found in annelida, arthropoda, and chordata. Each segment includes repetition of the same muscles, blood vessels, nerves, and/or reproductive structures. If one segment should fail, another could still function. Metamerism also allows diversification of body segments. Annelids are cephalized with a well-developed nervous system.
