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Trilobite

appendages, shield, arachnida, cephalic and thoracic

TRILOBITE, an early and primitive form of Crustacerce fossil, remains of which are very common in the Paleozoic era. They are doubt less the ancestors of modern Arachnida, that is spiders, scorpions and mites. In palmon tology, any individual of the order Trilobite, so called from the division of the external skeleton into three regions: (1) a cephalic shield; (2) a variable number of body rings and (3) a caudal shield, tail or pygidium. The cephalic shield is usually more or less semi circular, with an elevated portion, the glabella, bounded by the fixed cheeks, to which the free cheeks which bear the eye are attached by the facial suture. The posterior (genal) angles of the free cheek are commonly prolonged into longer or shorter spines. The eyes are sessile compound and consist of an aggregation of facets, covered by a thin cornea. The number varies greatly, Barrande having found as few as 14 and as many as 15,000 facets in each eye in different types. Behind the cephalic shield comes the thorax, composed of a number of segments (2 to 26), capable of more or less movement on each other; in several genera this freedom of movement was so great that species could roll themselves up into a ball, like a hedgehog. The tail is also composed of a number of segments (from two in Sao hirsuta to 28 in the genus Amphion), ankylosed or amal gamated. The extremity is sometimes rounded, but may be prolonged into a spine and the ends of the pleurae of the tail-segments may also be produced into spine-like processes. With re gard to the under-surface and appendages of the trilobites much remains to be discovered.

The head bears a hypostome or plate in front of the mouth and four pairs of jointed ap pendages, the basal parts of which were modi fied to serve as jaws.

From Walcott's examination of sections of rolled-up specimens, it appears that the thoracic appendages were slender, five-jointed legs, in which the terminal segment formed a pointed claw, and the basal segment carried a jointed appendage, homologous with the epipodite of many recent crustaceans. On each side of the thoracic cavity was attached a row of bifid, spiral branchial appendages, and appendages serving also as gills were probably attached to the bases of the thoracic limbs.

Trilobites vary greatly in size, some being scarcely larger than a pin's head, while species of Asaphus have been discovered two feet in length. They appear to have lived on muddy bottoms in shallow water, feeding on small marine animals, and probably swam on their backs, as do the recent Apus and the larval forms of Limulus. The general opinion of zoologists at the present time is that they are related, through the Xiphosura and Limulus, to the Arachnida, and are not crustaceans as for merly believed. Trilobites are characteristic of the Palzozoic system or rocks, and reached their maximum development in the Ordovician. The genera are numerous. (See ARACHNIDA), Consult Zittel and Eastman, (Text-book of Paheontology)• Walcott, 'Bulletin Museum Comparative Zoology> (Cambridge 1881).