What renders these animals peculiarly interesting is the circumstance that they are the only living representatives of a class which once filled in countless numbers the bosom of the primeval ocean, and whose fossil remains (Orthoceratites, Ammonites) furnish the naturalist with a series of historical documents, attesting the unmeasured age of our planet. What are the ruins, thirty or forty centuries old, that speak of the vanished glories of extinguished empires to these wonderful medals of creation that lead our thoughts through the dim vista of unnumbered centuries to the fathomless abyss of the past. * * * * * In point of development of organisation the Gasteropods or snails rank immediately after the Cephalopods. They also have a head plainly distinguishable from the rest of the body, and to which two brilliant black eyes give an animated expression. But their nervous system is far less developed, and while the lively cephalopod is able to swim about, and rapidly to seize a distant prey, almost all the gasteropods creep slowly along upon a flat disk or foot situated below the digestive organs, a formation to which they owe their name of gasteropods or stomach-footers. The marine snails are divided into several groups according to the different position and arrangement of their gills. In some species these organs form naked or free-swimming tufts on the back (Nudibranchiata) but generally they are variously disposed either in special cavities or under the folds of the mantle. Thus in the Inferobranchiata they are arranged under its inferior border on both sides of the body, or upon one side only, while in the Tectibranchiata they are placed, as in the Nudibranchiata, upon the dorsal aspect of the body, but are protected by a fold of the skin. In the Cyclobranchiata they form a fringe round the margin of the body, between the edge of the mantle and the foot, and in the Scutibranchiata and Pectinibranchiata they are pectinated, or shaped like the teeth of a comb, and placed in a large hollow chamber, which opens externally at the side of the body or above the head. [Illustration: Tiara.] [Illustration: Glaucus.] [Illustration: Scyllæa.] Nothing can be more elegant or various than the form and arrangement of the gills in most of the nudibranchiate gasteropods. In the Glauci and Scyllææ, we see at each side of the elongated body long arms branching out into tufty filaments; in the Briarei a hundred furcated stems serve for the aëration of the blood. On the back of the Eolides the gills are arranged in rows; in the Dorides they form a wreath or garland round the posterior intestinal aperture.