During the Mesozoic era the highest type of cephalopod, represented by modern squids and so-called “cuttlefishes,” branched off and developed in great profusion. These had slender internal shells, but no external chambered shells. An inky black liquid secreted in a bag was forced out to cloud the water when the animal was escaping its enemy, thus antedating by millions of years the principle of smoke screen so effectively used by ships during the World War. Some Jurassic species got to be over two feet long, and a few specimens of that age have been found in such perfect state of preservation that drawings of the fossils have actually been made with the ink (after moistening) taken from their own ink bags.
Fig. 54.—A fossil nonchambered cephalopod of Jurassic age. It was closely related to the modern squid, and its ink bag is well shown just to the left of the middle. (Modified after Mantell.)
Before concluding this chapter we shall take up the salient points in the geological history of Arthropods which constitute the highest subkingdom of all animals except the Vertebrates. They are now very abundant and varied, familiar examples being crabs and insects. A few scant remains of simpler forms are known from the Proterozoic, but since very early Paleozoic time they have been very common and have undergone great evolutionary changes. A few striking examples only will be dwelt upon. Among the most common and interesting of all Paleozoic animals were the trilobites, distantly related to modern lobsters and crabs.
Fig. 55.—Restorations of trilobites based upon actual fossils characteristic of earlier Paleozoic time: a, Cambrian; b, Ordovician; c, Devonian; b shows the appendages.
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.)