Trilobita

Trilobita, an order of fossil crustacea entirely confined to the Palæozoic rocks. They make their first appearance in the Cambrian, attain a great development in the Silurian, occur sparingly in the Devonian and the Carboniferous, and disappear finally in the Permian system. The dorsal surface of the body was covered with a calcareous shell or crust, which is usually trilobed longitudinally—i.e. the lobes extend from head to tail. This trilobed crust consisted of a cephalic shield, a variable number of movable body rings or thoracic segments, and a tail or pygidium, composed of a number of segments more or less anchylosed. The eyes were invariably sessile and compound, consisting of an aggregation of facets, covered by a thin cornea. The lenses are frequently well preserved, and in some species can readily be seen with the naked eye. Although usually faceted, yet in certain forms the eyes are smooth; while a few species had no organs of sight. The number of facets varies considerably. Thus, while in some types there are not more than fourteen facets, in others there are as many as 15,000 in each eye. The thoracic segments, which vary in number in the different types, were capable of more or less movement, and this to such an extent in some species as to allow of the animal rolling itself up like a hedgehog. Specimens showing the under surface are very rare, but the few which have been found show that the margin of the dorsal crust was turned under in the form of a more or less narrow 'doublure.' In the centre of this doublure in the cephalic region is attached the lip-plate or 'hypostome,' with which, as shown in one specimen, buccal organs (maxillary palpi) seem to have been associated. Other specimens have shown that all the segments of the thorax and tail carried jointed limbs, and that branchial filaments were also present. More recently Mr Walcot, by making thin sections of rolled-up specimens of trilobites, has greatly increased our knowledge of their under surface. He shows (in the case of Calymene) that the mouth, behind the lip-plate, was bounded by four pairs of jointed appendages, the basal joints of which acted as jaws; while the thoracic and pygidial segments each carried a pair of slender jointed legs. To the thoracic segments were likewise attached spiral gills, one row on each side; and probably branchial appendages were also attached to the bases of the thoracic legs. The sexes are believed to be indicated by variations in the breadth of the cephalic shield, and in the length of the cephalic and pygidial spines. The members of the order varied greatly in size, some species being scarcely larger than a pin's head, while others, like Asaphus gigas, attained a length of 18 inches, and even sometimes of 2 feet. The eggs of trilobites appear to have been deposited in clusters, but very little is known as to the larval condition of these curious animals. It is possible, as some think, that the young in many species may have been naked, but the dorsal crusts of minute larval forms have been met with, and it is not improbable that many so-called species may be only larval or transition forms of others. Trilobites appear to have swarmed on the muddy bottoms of Cambrian and Silurian seas, although they lived also in regions where only pure limestone was being accumulated. It is now believed that many of the peculiar markings seen upon the surfaces of Cambrian and Silurian strata, and some of which have been described as fossil plants, are really the tracks and trails of trilobites and probably other crustaceans. Some doubt exists as to the systematic position of Trilobita, but they are usually regarded by geologists as entomostracous crustaceans having relations to the phyllopods, but being more nearly allied to the Xiphosura (king-crabs). Hence some writers would include them under the Arachnida.