The valves of the shell are unequal, and are not placed respectively on the right and left sides of the body of the animal, but rather on the dorsal and ventral or upper and lower sides. The ventral shell is the larger, and is produced into a beak which sometimes has a round hole corresponding in position with the hole for the wick of an antique lamp, and the dorsal or smaller valve is always imperforate. The hinge is a perfect one, the junction of the two valves being so well secured by it that it is impossible to separate them without injury. It is formed by two curved teeth on the margin of the ventral valve that fit into corresponding sockets on the dorsal. A few brachiopods, however, have no hinge, the valves being secured by means of numerous muscles. The hole in the shell serves for the protrusion of a pedicel or foot by means of which the animal is enabled to attach itself. Two long arms, covered with vibratile cilia, and capable of being folded or coiled, are attached at the sides of the mouth. They are practically processes of the lips, mounted on muscular stalks, and attached to a delicate calcareous loop on the dorsal valve; and serve not only to produce water currents for the conveyance of food to the mouth, but also answer the purpose of gills. The digestive system of a brachiopod includes an œsophagus that leads into a simply formed stomach round which is a large digestive gland. The heart has only one cavity, but the animal is provided with two smaller and separate organs that assist in the propulsion of the blood, which circulates through numerous blood spaces in the bristly mantle. About two thousand fossil species of brachiopods are known, extending over a vast range of time; and the living species, numbering less than a hundred, are found from shallow water to the greatest habitable depths. Since the reader is hardly likely to form any extensive acquaintance with the Brachiopods, we shall illustrate our remarks by the introduction of only one species--the Serpent’s Head Terebratula (_Terebratulina caput-serpentis_), which is found in deep water in the North Sea. The interior of the dorsal valve, showing the calcareous loop above referred to, is represented in fig. 154, as is also the exterior of the shell, which is finely striated. The latter represents the dorsal aspect of the shell in order to show the hole in the upturned beak of the ventral valve. [Illustration: FIG. 155.--UNDER SIDE OF THE SHELL OF _Natica catena_, SHOWING THE UMBILICUS; AND OUTLINE OF THE SHELL, SHOWING THE RIGHT HANDED SPIRAL]