The sepia, or inky liquid, which all the Naked Cephalopods possess as a means of defence, is secreted in a pyriform bag, which has an outlet near the respiratory siphon. If the animal be alarmed when devouring its prey, it instantly lets go its hold, discharges the inky liquid into the water, and escapes unseen. The skin of this class of animals is thin and semi-transparent; the surface immediately below it consists of numerous cells, of a flattened oval or circular form, containing coloured particles suspended in a liquid. The colour is seldom the same in all these cells; the most constant kind corresponds with the tint of the inky secretion. In the Sepia there is a second series of cells, containing a deep yellow or brownish colour; in the common Calamary, or Squid, there are three kinds of coloured cells—yellow, rose-coloured, and brown; and in the Poulpe there are red, yellow, blue, and black cells. The cells possess the power of rapid contraction and expansion, by which the coloured liquid is drawn into deeper parts of the surface, and is again brought into contact with the semi-transparent skin—thus constantly varying. In consequence of the high development of the nervous system, the skin of the Naked Cephalopods is of extreme sensibility; a mere touch brings a blush on that of the Poulpe, and they all assume the colour of the surface on which they rest as readily as the chameleon. Many of these nocturnal animals are luminous, and are easily attracted by bright metallic objects. RECAPITULATION. NUMEROUS instances of microscopic structure may be found in the vertebrate series of marine animals, but the field is too extensive for the Author to venture upon. In the first section of this book, an attempt has been made to give some idea of the present state of molecular science—far short, indeed, of so extensive a subject; yet it may be sufficient, perhaps, to show the views now entertained with regard to the powers of nature, the atoms of matter, and the general laws resulting from the phenomena of their reciprocal action. By spectrum analysis it has been shown that not only many terrestrial substances, in a highly attenuated state, are constituents of the luminous atmospheres of the sun and stars, but that the nebulæ in the more distant regions of space contain some of the elementary gases of the air we breathe. In the succeeding sections it has been proved that the atmosphere teems with the microscopic germs of animal and vegetable beings, waiting till suitable conditions enable them to spring into life, and perform their part in the economy of the world. The life history of the lower classes of both kingdoms has been a triumph of microscopic science.