(Chapter 5) Footnote 8:

Vesalius cites De usu partium, Book 7; the passage he has in mind is in the eighth Book (3.631.2ff.): “If we find which of the parts situated in the head is lacking in the breast of those animals that do not have a head, it would not be unreasonable to say that this is the part for the sake of which the head was formed ... In the Carcini (crabs), Phalainae (moths), Carabi (spiny lobsters), and all animals without heads the eyes are on long stalks, etc.” (Tr. May 1968 pp. 395f.) The opening sentences of the present chapter are an extended paraphrase of this passage in Galen.