 The lack of names for animal kinds is also noteworthy; as in the corresponding passage from HA II, fish are mentioned once as an instance of blooded animals that lack the organs in question; but the stress throughout is on this particular system of organs in whichever blooded animals have them. Aristotle’s goal is to explain why it is that certain animals have these organs, and why it is that, if they have one of these organs, they must have the others. Some, but not all, of the explanations specify the function for the sake of which the parts are present; and even the material nature of the tissues, whether fleshy or sinuous, may be explained in functional terms. This discussion also displays a hierarchy of teleological connections of the sort described at the close of PA I 5, 645b28–33—one activity is present for the sake of another more basic one; a part that performs a subordinate activity is present for the sake of a part that performs a more important activity. Inhaling and exhaling are for the sake of respiration (which is for the sake of cooling the blood); a windpipe is present for the sake of transporting air to and from the lung; a neck is present to protect the windpipe.