Some of these grew to be two feet long, but usually they were only one or two inches long. First known from the earliest Paleozoic, they reached their culmination relatively early in the era and then dwindled away to utter extinction before its close. "They were characteristic of the Paleozoic era, beginning in great variety in the Lower Cambrian and dominating the seas of the Cambrian (300 species) and Ordovician (950 species). In the Silurian, though they were still common, the trilobites were nevertheless on the decline (485 species), and this ebbing of their vital force is seemingly shown in many picturesque forms replete with protuberances, spines, and exaggeration of parts. As a rule, in evolution, one finds that when an organic stock is losing its vital force there arises in it an exaggeration of parts, as if heroic efforts were being made to maintain the race. Spinosity in animals is often the prophecy of tribal death. In the Devonian, the variety and number of the trilobites were greatly reduced (105 species), at a time when the ancient types of fishes, which undoubtedly fed on these crustaceans (trilobites), began to be common in the seas. In the later Paleozoic seas, the trilobites were relics, or animals surviving from a time better suited to their needs, and one by one they vanished, until a little before the close of the Paleozoic era none were left." (Schuchert.) [Illustration: Fig. 56.--A giant, sea scorpion of Devonian time. Length nearly 3 feet. (After Clarke and Ruedemann, New York State Museum.)] An extraordinary type of Arthropod which ranged throughout Paleozoic time and became extinct at its close was the so-called "sea scorpion," closely related to the modern scorpion. Their five or six pairs of appendages all came out from the head portion, one pair in some cases having been developed as powerful pincers. Their culmination in size was reached during the Devonian when some forms grew to the astonishing length of over eight feet! Such gigantic creatures must have been tyrants of the seas until they were subdued by the oncoming powerful fishes. True scorpions are known from rocks as old as the Silurian. Lobsters and crabs made their appearance during the Mesozoic era.