^Suture^: The line of junction of the successive whorls of a univalve shell, or the line of closure of the opposite valves of a bivalve shell. ^Teeth^: Tooth-like projections on the hinge of a bivalve shell. ^Testaceous^: Having a hard shell, as distinguished from crustaceous, or soft-shelled. ^Turbinate^: Shaped like a whipping-top; whorled from a broad base to an apex. ^Umbilicus^: A central, navel-like depression in the body-whorl; a circular, more or less central pit or hollow. ^Umbo^: The beak of a bivalve shell; the protuberance of each valve above the hinge. ^Varices^, plural of ^Varix^: Raised marks on the shell denoting a former position of the lip of the aperture, and indicating periods of rest. ^Varicose^: Having prominent and tortuous formations on the shell. ^Ventricose^: Having the whorls of the shell swollen or strongly convex. ^Whorl^: One of the turns of a spiral shell. /Body-whorl/, the last whorl. [pg305] MOLLUSKS We now come to another large and important group of animals, with characters so peculiar to itself and differing so radically from those which distinguish any other class of living creatures, that it may be said to occupy almost a unique position in the animal kingdom. Recent investigations have demonstrated that the larval form of mollusks presents some remarkable points of similarity to the embryonic forms of the /Annelida/ and to the larvæ of some other classes of lower organisms. This discovery is one of the triumphs of embryology in its patient search for that connecting-thread that weaves together all the varying forms of animal life. Aside from these subtle evidences of relationship revealed by the microscope, the mollusks appear to occupy a position of considerable isolation in the biological world. As accepted by zoölogists to-day, this phylum is but the remnant of its former self. Aristotle considered all creatures with a testaceous covering to belong to a single family, and those later patriarchs of biology, Linnæus, Cuvier, and Lamarck, extended the group to include the greater part of all the marine invertebrate animals. Little by little the phylum has been shorn of orders and classes. First, the worms and the /Echinodermata/ were separated into distinct phyla; then the barnacles were discovered to be crustaceans, and were accordingly removed from their position as "multivalves" under the /Mollusca/; then the tunicates, or ascidians, were found, through the critical examination of their larval stage, to be merely masquerading as mollusks; and lastly, the brachiopods have been somewhat reluctantly removed from their old position with the mollusks and given the [pg306] distinction of a phylum almost exclusively to themselves: they retain their old associations only in the name of /Molluscoida/.