A reader wrote us the following brief message: “Do worms have platelets in their blood? It’s a simple question, but I can’t seem to find the answer anywhere.” It is in fact a simple question, at least in form. However, seemingly basic questions are often hard to answer, particularly when they require a fairly advanced understanding of worm physiology. In this instance, though, we were able to find the answer with relative ease: no, worms do not have platelets in their blood.
We are tempted to leave the matter there, but we thought it might be worthwhile to explain how we arrived at this conclusion, as well as explain some of the terms involved in this question. Taking up the latter first, a platelet is simply a blood cell that functions to stop bleeding. They are quite small – for a point of reference, they are about 20 percent the size of a red blood cell – and like red blood cells, they lack nuclei. The reader specifically asked about the blood of “worms,” and what exactly are worms? As we explain in the article just linked to, the term is somewhat complex because of the loose usage that surrounds it. We see no reason to rehash that discussion here, and moreover it isn’t really of concern to our reader. When she used the word “worms,” we presume she meant an earthworm, as this is what most people mean when they refer to a generic worm. However, even if she wasn’t referring to earthworms, no creature that might be referred to as a “worm” has platelets.
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How do we know this? It turns out that only mammals have platelets. It isn’t known with certainty why this is the case, but a promising hypothesis is that platelets evolved to counteract the risk of bleeding to death during mammalian childbirth. (Lethal hemorrhaging during childbirth is a threat only to mammals.) Since worms obviously aren’t mammals, they do not have platelets in their blood. They would serve no function in the comparatively simple closed-blood circulatory systems found in earthworms.
So, in this instance, we can respond to our reader’s question with a simple answer with no qualifications: worms do not have platelets.
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