Having now taken _H. aspersa_ as the representative of our univalves, let us take the "Pearl Mussel"--_Unio margaritifer_--as that of our bivalves, all of which live in the water, whereas of univalves some are "land snails" and some "water snails." It would say of itself, "I am a species of the genus Unio (_unio_, a pearl), which belongs to the family Unionidæ, which belongs to the sub-order Isomya (_i.e._ having muscles of equal power to close the two valves of the shell), which belongs to the order Lamellibranchiata (_i.e._ having gills arranged in leaf-like fashion), which belongs to the sub-class Pelecypoda (_i.e._ having a foot somewhat of an axe-shape), which belongs to the class Acephala (headless), which is the second of the two chief classes into which Mollusca are divided.
"I differ from the Gasteropoda (whether they be terrestrial or aquatic) in that I and my near relations are exclusively aquatic and of a sedentary life, which makes the protection of two encompassing shells necessary. These shells are secreted by my mantle lobes, and are united by a ligament which tends to make the valves 'gape' for water and food and by two contracting muscles which close them in danger. I have a degenerate brain and no eyes. My mouth has neither jaw nor teeth, but possesses nervous lips covered with cilia, the vibration of which carries food-laden water to my mouth. My foot, when protruded, is seen as a large muscular appendage, and, by alternately expanding and contracting, it enables me to burrow or plough through mud or even sand, and so disturb the minute organisms on which I feed. I can thus travel fifteen feet a day, or about a mile in a year.
"I have no eyes, but distinguish well between light and shade by means of the surface of my body when exposed. I breathe, that is, get oxygen from the water, by means of gill-plates. As regards other internal organs, I differ not much from _H. aspersa_, but I am either male or female. Outside I am black and uncomely; but within I am pearly-white, and but for my power of forming pearls round an irritating grain of sand the civilization of England would have come to pass later than it did, for it was the report of my pearls which brought Cæsar to Britain."