The name, arm-footed, was given them in reference to a pair of special structures called the arms, bearing a large number of tentacles; it is now more frequently spoken of as the lophophore (see p. 122), and regarded as comparable to the lophophore of the Polyzoa, spread out into two portions. With the latter group the Brachiopods were formerly united by Huxley, under the name of Molluscoidea. This name is now obsolete, because it is understood that all these creatures are widely different from Molluscs; but the theory of relationship of the Brachiopoda to the Polyzoa, implied in it, still holds good. The chief importance of this group lies in its fossil forms, which are exceedingly numerous, particularly in the Mountain Limestone of the Carboniferous Period; it is crowded with their shells, especially a form named, from its elongated shape, Productus. The shells of Brachiopods are equal-sided; that is to say, the right and left valves match; but they are inequivalve, the ventral valve being much the biggest. It often contains a foramen, or hole, at the beak, for the passage of the pedicle, or stalk, by which the animal is attached to the ground (_e.g._ _Terebratula_, _Rhynchonella_). Sometimes, however, the pedicle passes out between the valves (_Lingula_, _e.g._), in which case there is no foramen; or it may be arranged in other ways. Sometimes the shell is merely attached to the ground by its side, like an oyster. Some forms are enormously widened in a lateral direction, _e.g._ _Spirifera_, and the _Productus_ above named. _Lingula_, among others, is remarkable as being a form that has survived from the earliest geological period to the present day. TABLE SHOWING THE CLASSIFICATION OF THE BRACHIOPODA { TESTICARDINIS.--Shell Calcareous, with hinges. { Skeleton present in the arms. =BRACHIOPODA.= { { ECARDINES.--Shell comparatively soft, composed { of Chitin, only strengthened by deposits { of lime, without hinges. No skeleton in { the arms. The larva, in its best known forms, passes through the typical larval stages of the animal kingdom. It is first a one-layered larva, then a two-layered form, and then becomes a ciliated animal. In this three regions may be distinguished, representing respectively the head, body, and pedicle. The shells of the Brachiopoda, including the kinds above named, may be seen by the reader in any geological museum. CHAPTER XI THE POLYZOA; MOSS-CORALS AND SEA-MATS