According to most scholars, in the Parts of Animals Aristotle frequently provides explanations in terms of material necessity, as well as explanations in terms of that-for-the-sake-of-which, i.e., final causes. In this paper, I argue that we misunderstand both matter and the way that Aristotle explains things using necessity if we interpret Aristotle as explaining things in terms of material necessity. Aristotle does not use the term “matter” very frequently in his detailed discussions of animal parts; when he does use it, he typically identifies blood as matter. I argue that this is because blood is, for Aristotle, what properly nourishes and grows the other parts of the body and he views nourishment and growth as types of coming-to-be. The second half of the paper turns to necessity as a cause in the Parts of Animals. I argue that in the Parts of Animals Aristotle is not interested in distinguishing between what Aristotle elsewhere treats as very different types of necessity.