36 3. Cross Section: Obtain a prepared slide from the side bench, and examine a cross section of the earthworm showing the setae, and identify the following structures or tissues on your slide, in comparison with the description and illustration below. • In the body wall , identify the external cuticle , the cellular epidermis beneath it, a thin layer of circular muscles and a thick layer of longitudinal muscles . (Why do you think the longitudinal muscles are so much more heavily developed?) • The innermost surface of the longitudinal muscles is covered by a thin peritoneum (you may have to examine the section at 400x to observe it). • Penetrating through the body wall are 4 pairs of setae : 2 lateral pairs and 2 ventrolateral pairs. Can you find the muscles that protract and retract the setae? • Locate the intestine , and note the presence of a large rod of tissue hanging suspended in it -- the typhlosole . Notice the lumen of the intestine is made quite narrow by the typhlosole, which greatly
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