Lobsters vary a good deal in colour, but as a rule a living Lobster is of a more or less mottled dark blue, becoming nearly black on the back, and shaded off into orange yellow or red on the under-side. This coloration resides in the shell, and does not change much after the shell has hardened. In this respect the Lobster is unlike many of the smaller Crustacea which have a thin and more or less transparent exoskeleton, and in which the colour resides in certain living cells (chromatophores) of the underlying skin. Many of these Crustacea possess the power of changing their colours to a remarkable degree, by the expansion and contraction of the branched chromatophores. The question which is often asked, "Why does a Lobster turn red when it is boiled?" is one to which it is not easy to give a simple answer. A chemical change takes place under the influence of heat in the pigment of the shell, which changes it from blue to red; how slight the change is, is perhaps shown by the fact that occasionally living Lobsters are found of a red colour almost as brilliant as that which is assumed on boiling. [Illustration: FIG. 9--SIDE-VIEW OF ROSTRUM OF (A) COMMON LOBSTER (_Homarus gammarus_) AND (B) AMERICAN LOBSTER (_Homarus americanus_)] The Common Lobster is found on the coasts of Western Europe, from Norway to the Mediterranean, living in shallow water, generally a little way below low-tide mark, wherever a rough, rocky bottom affords suitable lurking-places. On the Atlantic coast of North America, Lobsters are also found abundantly in similar situations. These American Lobsters, if examined carefully, will be found to differ from the European kind in certain small details of structure, of which the most conspicuous is the presence, on the under-side of the rostrum, of two spines or teeth. In the European Lobsters the under-side of the rostrum is smooth (Fig. 9). In the nomenclature of technical zoology, these two kinds or _species_ of Lobster are said to constitute (along with a third species found at the Cape of Good Hope) the genus _Homarus_, the European species being known as _Homarus gammarus_, and the American as _Homarus americanus_. The so-called "Norway Lobster" or "Dublin Prawn," which differs from the Common Lobster in having large kidney-shaped eyes and long and slender claws, and in many other details of structure, is placed in a distinct genus, and is known as _Nephrops norvegicus_. The genera _Homarus_ and _Nephrops_, together with some others, constitute the family Homaridæ, which again is grouped with other families in a tribe, Nephropsidea, forming a part of the order Decapoda. These groups are intended to express the varying degrees of resemblance and difference in structure between the species of animals which make up the class Crustacea. Since we have good grounds for believing that all these species have arisen by some mode of evolution, this classification also represents the varying