Fig. 141.—Section of Skin and Underlying Tissues in the Head-Region of Ammocœtes. "a", cranial wall; "b", muco-cartilage; "c", laminated layer; "d", external cuticular layer. (All references in this work to "Ammocœtes" or "Petromyzon" appear to refer to the European Brook Lamprey, now called "Lampetra planeri".)