 On the Parts of Animals II-IV presents Aristotle’s attempt to provide causal explanations for the sort of information one finds organized in HA I-IV, information about the uniform and non-uniform parts of animals. (PA I, it will be recalled, is a discussion of the norms that are needed to guide the scientific investigation of animals; see section 4, above.) Here, I will be narrowly focused on just a couple of passages in PA II-IV, but will begin with a very brief sketch of its overall structure. It begins (PA II 1–2) with a careful and complex discussion of the causal relationships between uniform and non-uniform parts (roughly, this distinction corresponds to our distinction between bodily fluids and tissues and organs), stressing the teleological priority of the whole organism to the parts, and among parts the priority of the non-uniform to the uniform. PA II 3–10 then discusses the uniform parts, beginning with that out of which all the others are nourished, blood, and ending with flesh, bone, and analogous materials (nail, horn, hoof). From II 10 to III 3, Aristotle discusses the external parts of blooded animals associated with the head such as eyes, ears, nose and mouth. But when he gets to the neck, he turns to the internal parts, and from III 3 to IV 4 he discusses the ‘innards’ of the blooded animals, parts such as heart, lung, liver, kidneys, and so on. From IV 5 to 9 he discusses the parts, both internal and external, of the ‘bloodless’ animals, i.e. the crustaceans, testaceous mollusks, cephalopods and insects. Finally, at IV 10, he returns to the external parts associated with the torso and limbs of the live bearing and egg laying quadrupeds (10–11), birds (12), and fish (13); and ends by discussing a number of animals that ‘dualize’ between being land-dwellers and water dwellers or flyers.