Trilobites

Trilobites. an order of Arthropoda (q.v.), all the members of which are extinct and limited to the Palaeozoic rocks. The order is of interest from its importance to the geologist, the variety of forms belonging to it, and the doubts as to its correct zoological position. The body consists of three divisions, a head, thorax, and tail or abdomen, which are distinctly marked off from one another. It is distinctly trilobed in most genera, there being a raised central area with a lower flat area on either side. The thorax is always composed of a number of distinct somites, varying from two in the small Agnostus to as many as twenty-six in Harpes; each somite consists of a central raised axis with a rib-like pleura on either side. The head or cephalic shield is composed of several somites fused together; the central portion or "glabella" is raised, and covers the stomach; on either side are two pairs of plates called the fixed and free cheeks. The free cheeks are often continued backward into long spines as in Trinucleus. On the upper surface of the head are usually a pair of eyes; these are sometimes compound and of great power, and in some there may be as many as 15,000 facets in each eye. On the under surface of the head is a lip-plate or hypostome much like that of Apus. The tail or caudal shield or "pygidium" consists of several somites fused into a single plate; it often ends in a point or spire; the anus opens at the extreme end. As regards the structure of the soft parts and organs on the under surface very little is known. As the Trilobites could mostly roll themselves up into a ball, like the wood-louse, the appendages are sometimes preserved, but so much displaced that their interpretation is very difficult. It is thought, however, that one or two pairs of spiral gills occurred on each somite under the pleurae of the thoracic region, and that each somite had also a pair of crawling legs with an epipodite or small-jointed appendage attached to each. The systematic position of the Trilobites is very uncertain. They have been most generally regarded as Crustacea, though many weighty authorities assign them to the Arachnida. The latter view is based on their affinities to the Limulus, which is now generally regarded as an Arachnid. During the last few years, however, the balance of evidence seems swinging back to the Crustacean view of their affinities, owing to the growth of knowledge of the appendages and the position of the anus. In this case probably their nearest living ally is the genus Apus, one of the Phyllopoda. The Trilobites are restricted to the Palaeozoic rocks, and range from the Cambrian to the Permian; in the Carboniferous they are represented by four genera, while in the Permian there is only a single species of Phillipsia. They are most important in in the Upper Cambrian and Ordovician. Many of the best known forms, such as Calymene blumenbachi or the "Dudley Locust," occur in the Wenlock Limestone. The oldest known genera are Olonellus and Paradoxides. They are classified into nineteen families.