Brachiopods are called lampshells because they resemble ancient oil lamps. Like clams and other bivalve molluscs (Phylum Mollusca), the brachiopod has two apposed hard shells (valves). The soft body of a brachiopod is enclosed by dorsal and ventral tissue folds (mantle). The mantle secretes the pair of valves, which are covered by epithelial tissue called the periostracum. The bilaterally symmetrical valves that make up the shell are typically dissimilar. They range in size from 2 mm to nearly 100 mm in shell length and are usually cemented to the substrate by a stalk called the pedicle. Lacazella and Crania attach directly to the substrate by the ventral shell. Brachiopod symmetry differs from that of clams; the pedicle valve of brachiopods is ventral and the other, smaller shell is dorsal, whereas the shells of bivalve molluscs are arranged on the left and right sides. The anterior part of the space inside the brachiopod shell is occupied by the tentacle-bearing lophophore, which functions as a surface for gas exchange and food getting. The name Brachiopoda stems from the "arms" of the lophophore. Unlike molluscs, the brachiopod has a lophophore; as a consequence, brachiopods are grouped with other lophophorates - bryozoans (ectoprocts; Phylum Bryozoa) and phoronids (Phylum Phoronida). The brachiopod body occupies the posterior part of the space inside the shell.