Final answer:
Blood vessels play a crucial role in temperature regulation through the skin of multicellular organisms.
Step-by-step explanation:
In a model to illustrate how multicellular organisms regulate their temperature through their skin, blood vessels can be included to represent the role of the circulatory system in temperature regulation.
The blood vessels in the skin help to regulate temperature by constricting or dilating, which controls the flow of blood to the surface of the skin. When the body is too hot, the blood vessels dilate to allow more blood flow and heat loss through the skin. When the body is too cold, the blood vessels constrict to minimize heat loss through the skin.
The other options, such as fingernails, teeth, and stomach, are not directly involved in temperature regulation through the skin.
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