Skins and Skeletons Help
Introduction to Skins and Skeletons
Having introduced and characterized the mammals, it is now appropriate for us to further examine the meaning of the following claim: “Oh, those pesky animals! They’re just a bag of bones!” Yes, this is true! The “bag” is their surrounding skin, while the “bones” are the major organs making up their skeletons.
Yet, there is also another reason why we consider both the skin and the skeleton together. Recall that in all animals, except for sponges, there are two or more germ layers within the embryo. These germ layers are technically called derms or “skins,” since they consist of thin groups of primitive cells that progressively differentiate ( dih -fer- EN -she-ate) or “become more different,” as the embryo undergoes its development. In the hollow gastrula and later stages of embryonic development, there are three derms or skins. These are the endoderm (inner skin), mesoderm (middle skin), and ectoderm (outer skin).
The ectoderm eventually forms much of the skin, hair, nails, the enamel of the teeth, and the most important portions of the nervous system. The mesoderm, in contrast, ultimately gives rise to the bones, muscles, much of the heart and circulatory system, and the connective tissues. We consider skin and skeleton together in the same chapter because they arise from two adjacent germ layers within the embryo.
But the final reason we lump them together is due to the answer to the following question: “When you are out walking in the woods or fields, and you come upon the remains of a long-dead animal, what part of the body is usually left for you to identify?” Certainly, the two most prominent (and perhaps toughest) parts of the dead animal are its covering of skin (with maybe attached fur or feathers) on the outside, and its bony skeleton lying within. Indeed, paleontologists (scientists who study ancient life) most frequently use skeletal remains – teeth, and (if they’re very lucky) fragments of preserved hide or skin — to help them identify and classify long-dead or extinct animals. Thus, we study skin and bones together, because that’s all what is usually left after an animal dies!
Practice problems for these concepts can be found at: Skins And Skeletons Test
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