Exploration the Structure of Animals Essay

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Structure of Animals

Two animal phyla that can be compared and contrasted are Nematoda and Annelida. Nematoda are roundworms. There are more than 15,000 known species, with scientists estimating there may be as many as half a million species yet to be discovered (Waggoner, 2009). Nematoda have a worm-like appearance outside and a simple internal body structure. The phylum Annelida includes earthworms and their relatives, leeches, and a large number of mostly marine worms known as polychaetes (Waggoner, 2006). Annelids are characterized by their segmented bodies. They also have bristles on their bodies. It is estimated there are approximately 9,000 species of annelids today.

The Animal Phylogenetic Tree ("Mastering Biology," 2012) is a useful visual learning aid that is helpful in demonstrating the similarities and differences between these two phyla. The first diagram shows eight phyla, of which Nematoda and Annelida are two. Only one of the phyla, Porifera, is not characterized by symmetry.

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Cnidaria and Echinodemata are two phyla on the tree that have radial symmetry. The starfish is one commonly known example. Any of the starfish arms could be cut through the center, producing two equal halves. Nematoda and Annelida are both symmetrical, and one half mirrors the other.

Porifera is also the only phyla in which animals lack true tissues. An example of this group of animals is the sponge. They have a simple cellular level of organization and get food when water washes through the body. Nematodes and annelids, by contrast, have more complex organization. Like the phyla Echinodermata, Chordata, Mollusca, and Arthropoda, Nematoda and Annelida are triploblastic, meaning they have three germ layers. The layers are the endoderm (innermost layer), mesoderm (middle layer) and ectoderm (outer layer).

Annelids have a body cavity called a coelom, which is completely lined with mesoderm ("Mastering Biology," 2012). The body cavity is the fluid-filled.....

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