Fig. 271. Cambrian Inarticulate Brachiopods
A, Lingulella; B, Discina
Brachiopods. These soft-bodied animals, with bivalve shells and two interior armlike processes which served for breathing, appeared in the Algonkian, and had now become very abundant. The two valves of the brachiopod shell are unequal in size, and in each valve a line drawn from the beak to the base divides the valve into two equal parts ([Fig. 270]). It may thus be told from the pelecypod mollusk, such as the clam, whose two valves are not far from equal in size, each being divided into unequal parts by a line dropped from the beak ([Fig. 272]).
Brachiopods include two orders. In the most primitive order—that of the inarticulate brachiopods—the two valves are held together only by muscles of the animal, and the shell is horny or is composed of phosphate of lime. The Discina, which began in the Algonkian, is of this type, as is also the Lingulella of the Cambrian ([Fig. 271]). Both of these genera have lived on during the millions of years of geological time since their introduction, handing down from generation to generation with hardly any change to their descendants now living off our shores the characters impressed upon them at the beginning.
The more highly organized articulate brachiopods have valves of carbonate of lime more securely joined by a hinge with teeth and sockets ([Fig. 270]). In the Cambrian the inarticulates predominate, though the articulates grow common toward the end of the period.
Fig. 272. A Cambrian Pelecypod
Mollusks. The three chief classes of mollusks—the pelecypods (represented by the oyster and clam of to-day), the gastropods (represented now by snails, conches, and periwinkles), and the cephalopods (such as the nautilus, cuttlefish, and squids)—were all represented in the Cambrian, although very sparingly.
Pteropods, a suborder of the gastropods, appeared in this age. Their papery shells of carbonate of lime are found in great numbers from this time on.