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Animals in the Phylum Annelida include the earthworms, leeches, and many marine worms. These protostome coelomates exhibit true segmentation. Segmentation refers to body plans that are divided into discrete units, which may be repeating or may each have a unique function in the body.

It is easy to detect segmentation in earthworms. The segments are physically separated internally by thin sheets of mesoderm-derived tissue termed septa (singular: septum; recall, the term septa was also used to describe structures that separate cells in the hyphae of fungi, i.e., septate hyphae). Because of these separations, earthworms can contract muscles in some segments without affecting the hydrostatic pressure in adjacent segments. This ability is highly advantageous for movement, and through coordinated contraction and expansion of segments, earthworms are able to burrow into the soil with ease.

The figure above shows some of the internal segmentation in an earthworm. The bodies of most annelids consist of a series of repeating segments, with repetition of organ systems (muscular, nervous, reproductive, circulatory, and excretory) in the segments. Therefore, most annelids do not capitalize on one of the main benefits of segmentation, the ability to specialize. Note, many segments have multiple hearts, and each segment has a pair of nephridia (an excretory organ). Also note how the longitudinal blood vessels, nerve cords, and digestive tract all run the length of the body, whereas the pumping vessels and ganglia are all arranged in segments. In contrast, some arthropods show extreme specialization by exploiting segmentation.

Animals II Part 3 VoiceThread Transcript

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