In fact this simple type, almost exactly like its early Paleozoic ancestor, has been the only one out of this whole remarkable class of animals to persist to the present time, being now barely represented by the well-known pearly nautilus of the Indian Ocean. 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. [Illustration: 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. [Illustration: Fig. 55.--Restorations of trilobites based upon actual fossils characteristic of earlier Paleozoic time: a, Cambrian; b, Ordovician; c, Devonian; b shows the appendages.]