The _Brachiopoda_ are bivalve molluscs, but unlike most other molluscs they are rooted to one spot, and destitute of any power of locomotion. Their shells are unequal, the dorsal, or upper valve, being smaller and usually more bulged out than the under or ventral valve, which in most species is prolonged at its narrow end into a kind of beak. In the terebratula this beak has a little circular hole, from which there emerges a short peduncle or stalk, that fixes itself firmly to a rock or other substance at the sea-bottom, and serves the purpose of an anchor and cable to keep the little vessel safely moored. When the shells are detached, these perforated ventral valves have so exactly the form of the old Roman lamps, "that they were called _Lampades_, or lamp-shells, by the old naturalists."[36] Other species, as the _lingulæ_, have no beak, and the long peduncle passes out between the valves, which are of nearly equal size, and have been compared to the shape of a duck's bill. In yet another genus, the _crania_, there is no peduncle, but the animal adheres by its lower valve, much like the oyster, and may often be seen clustered in groups on decayed sea urchins or other organisms, particularly in the chalk formation. [Footnote 36: See the excellent _Manual of Mollusca_, by Woodward, p. 209.] The internal structure of these animals is singularly beautiful. The inner surface of each valve is lined with a soft membranous substance, called the pallial lobe, the margin of which is set round with stiff hair-like bristles, that prevent the ingress of any foreign body likely to interfere with the play of the delicate filaments of the arms. These two soft lobes are furnished with veins, and supply the place of a breathing apparatus. The body of the animal occupies not quite a third part of the interior of its valves, and is situated at the narrow end. There are thus two distinct regions within the shell, separated from each other by a strong membrane, through the centre of which is the opening of the mouth. The smaller cavity next the hinge contains the viscera, and the outer larger one, the folded and ciliated arms. These arms form one of the most characteristic features of the brachiopods. They are two in number, and proceeding from the margin of the mouth, advance into the outer empty chamber of the shell, and return upon themselves in spiral curves and folds. They are fringed with slim, flat, narrow filaments, set along the arm like teeth along the back of a fine comb. Though called arms, these long ciliated appendages are rather enormously protruded lips. The vibratory action of the fringes causes currents to set inwards towards the mouth, which is placed at the inner end or base of the arms. To support these long convoluted arms, many of the genera are furnished with slender hoops of hard calcareous matter, which are hung from the dorsal valve, and are still found within the shells of some of the most ancient fossil brachiopods.