Page:EB1911 - Volume 04.djvu/376

 In Terebratella the loop is attached to the hinge-plate and to the septum (fig. 17). In Megerlia it is three times attached, first to the hinge-plate, and then to the septum by processes from the diverging and reflected positions of the loop. In Magas the brachial skeleton is composed of an elevated longitudinal septum reaching from one valve to the other, to which are affixed two pairs of calcareous lamellae, the lower ones riband-shaped; attached first to the hinge-plate, they afterwards proceed by a gentle curve near to the anterior portion of the septum, to the sides of which they are affixed; the second pair originate on both sides of the upper edge of the septum, extending in the form of two triangular anchor-shaped lamellae (fig. 18). In Bouchardia the septum only is furnished with two short anchor-shaped lamellae. Many more modifications are observable in different groups of which the great family Terebratulidae is composed. In Thecidium (figs. 3,4) the interior of the dorsal valve is variously furrowed to receive the lophophore folded in two or more lobes.

In the family Spiriferidae there are two conical spires directed outwards, and nearly filling the cavity of the shell (fig. 5); while in Atrypa the broad spirally coiled lamellae are vertical, and directed toward the centre of the dorsal valve. In the Rhynchonellidae there are two short slender curved laminae, while in many genera and even families, such as the Productidae, Strophomenidae, Lingulidae, Discinidae, &c., there exists no calcified support for the labial appendages. The ventral valve in many of the genera is provided with two curved hinge-teeth, which fit into corresponding sockets in the opposite valve, so that the valves cannot be separated without breaking one of the teeth.

Each valve of the shell is lined by a mantle which contains prolongations of the body cavity. The outer surfaces of the mantle secrete the shell, which is of the nature of a cuticle impregnated by calcareous salts. These often have the form of prisms of calcite surrounded by a cuticular mesh work; the whole is nourished and kept alive by processes, which in Crania are branched; these perforate the shell and permit the access of the coelomic fluid throughout its substance. These canals are closed externally and are absent in Rhynchonella, where the amount of calcareous deposit is small. In Lingula the shell is composed of alternate layers of chitin and of phosphate of lime. The free edges of the mantle often bear chitinous bristles or setae which project beyond the shell. As in the case of the Lamellibranchiata, the shell of the adult is not a direct derivative of the youngest shell of the larva. The young Brachiopod in all its species is protected by an embryonic shell called the “protegulum,” which sometimes persists in the umbones of the adult shells but is more usually worn off. In all species it has the same shape, a shape which has been retained in the adult by the Lower Cambrian genus Iphidea.

The body of the Brachiopod usually occupies about the posterior half of the space within the shell. The anterior half of this space is lined by the inner wall of the mantle and is called the mantle cavity. This cavity lodges the arms, which are curved and coiled in different ways in different genera. The water which bears the oxygen for respiration and the minute organisms upon which the Brachiopod feeds is swept into the mantle cavity by the action of the cilia which cover the arms, and the eggs and excreta pass out into the same cavity. The mouth lies in the centre of the anterior wall of the body. Its two lips fusing together at the corners of the mouth are prolonged into the so-called arms. These arms, which together form the lophophore,