During the long ages of the Cambrian the trilobite varied greatly. Again and again new species and genera appeared, while the older types became extinct. For this reason and because of their abundance, trilobites are used in the classification of the Cambrian system. The Lower Cambrian is characterized by the presence of a trilobitic fauna in which the genus Olenellus is predominant. This, the _Olenellus Zone_, is one of the most important platforms in the entire geological series; for, the world over, it marks the beginning of Paleozoic time, while all underlying strata are classified as pre-Cambrian. The Middle Cambrian is marked by the genus Paradoxides, and the Upper Cambrian by the genus Olenus. Some of the Cambrian trilobites were giants, measuring as much as two feet long, while others were the smallest of their kind, a fraction of an inch in length. Another type of crustacean which lived in the Cambrian and whose order is still living is illustrated in Figure 269. [Illustration: Fig. 269. A Phyllopod] =Worms.= Trails and burrows of worms have been left on the sea beaches and mud flats of all geological times from the Algonkian to the present. =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). [Illustration: Fig. 270. A Cambrian Articulate Brachiopod, Orthis] [Illustration: Fig. 271. Cambrian Inarticulate Brachiopods A, Lingulella; B, Discina] 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.