TRILOBITA, a group of extinct Arthropoda of which the fossil remains are found in the rocks of the Palaeozoic era. Many species are found in the Lower Cambrian, among the earliest known fossils. They are abundant in the Ordovician and Silurian when they begin to decline and only a single genus survives in the Permian.
A typical trilobite has a segmented body roughly resembling that of a wood-louse, with the dorsal surface marked by two longi tudinal furrows which divide it into the three lobes alluded to in the name of the group. In the head-region the middle lobe is the glabella and usually shows transverse grooves marking the five segments of which the head is composed. On either side of the head are plates, the "free cheeks," divided from the central part of the head by the "facial sutures" and bearing the compound eyes. In some trilobites, however, the eyes are reduced or absent. Behind the head a number of the body-segments, from two to 29, are freely movable, and they are followed by a tail-shield or "py gidium" which often shows, by transverse furrows, that it is com posed of a number, sometimes a large number, of segments_ Some trilobites, such as Calymmene, could roll up into a ball like many wood-lice and are often found in this condition.
For long no definite traces of the limbs of trilobites could be discovered and their nature was the subject of much speculation. They have now been described in several genera by C. D. Walcott, C. E. Beecher and P. D. Raymond. The structure is most fully known in Triarthrus, investigated by Beecher and by Raymond. The appendages of the first pair, placed at the sides of the f ore lip (hypostoma) which underhangs the mouth, are long, slender, many-jointed antennae. The remaining appendages are all similar in structure and are attached, four pairs to the head and one pair each to the somites of the body whether free or coalesced into the pygidium. They are two-branched, the inner branch (endopodite) being a jointed leg, while the outer (exopodite) bears a fringe of what seem to be stiff flattened bristles. The two branches spring from a single basal segment drawn out on the inner side as a jaw lobe (gnathobase) which served, no doubt, to seize food and pass it forward to the mouth. Towards the hinder end of the body the appendages become smaller and the inner edge of the endo podite is lobed in a way recalling the appendages of the Branchio pod Crustacea. In Neolenus another point of resemblance to
certain Branchiopoda is provided by a pair of long thread-like tail filaments. Walcott believed the structure of the limbs to be more complex, certain genera possessing, in addition to the parts men tioned, two or three "epipodites," but the presence of these awaits confirmation. Still less probable is Walcott's conclusion that Calymmene and Ceraurus possessed corkscrew-like gills.
The development of various species has been traced. Starting with a minute larva with only indications of head and pygidium, segments appear between these regions in order from behind f or wards, the new segments being set free in succession from the front edge of the pygidium. The relationships of trilobites to the various groups of Arthropoda now living have been the subject of much discussion but the elucidation of their appendages leaves no doubt that their main affinities are with the Crustacea (q.v.). The five pairs of appendages on the head-region are directly com parable with those of Crustacea, since the second pair or antennae of Crustacea are still postoral and biramous and carry a mastica tory process or gnathobase in the nauplius larva as in the trilobites. The biramous form of all the postoral limbs is also a weighty argu ment in favour of crustacean affinity. On the other hand, there is no trace of the characteristic crustacean shell-fold or carapace. In the uniformity of the postoral appendages, however, the trilobites are more primitive than any living crustacean. The view that trilobites are phylogenetically connected with Arachnida (q.v.) has less to support it, but it is possible that connecting links may yet be proved to exist in the imperfectly known Cambrian Limu lava of Walcott.
The trilobites were all marine and lived, some on sandy or muddy bottoms, some on coral reefs, and some, perhaps, in the deep sea. They were distributed all over the world and in some localities were in such abundance that rock strata are crowded with their remains. A very large number of genera and species are known. Their range of size is from a quarter of an inch to coin. in length, but most species are between one and three inches long. (W. T. C.)