Page:The New International Encyclopædia 1st ed. v. 19.djvu/535

* TRILLIUM. 465 TRILOBITA. Maine to Florida. They have a naked stem, 4 to 15 inches high, terminated by three ovate, pointed, broad, beautifully netteil-veiued leaves. Trillium (jnmdifloruin, also called wake-roliin, is a pretty wild llower, growinjj in rich woods from Vermont to Kentucky, Wisconsin, and northward. The flowers, which appear in spring, are at first nearly white, but change with age to rose-color. They are borne on a peduncle from 2 to 3 inches long. TBILOBITA (Neo-Lat. nom. pL, from Gk. Tpe?s, treis, three + Xi/Sot, lobos, lobe). An important gi'oup of extinct marine crab-like ani- mals, whose fossil body shields are found in rocks of Paleozoic age in most parts of the world. The trilobites constitute a subclass of Crustacea, of which class they are very primitive members. The trilobite body was covered by a hard, calcareous dorsal shield and a thin ventral integument. The dorsal shield or carapace is of elliptical or oval outline and is divided by two dorsal furrows into three longitudinal lobes, the median axis and two lateral pleural lobes. This trilobate division, which characterizes this class of animals, gave the group its name. The shield is also divided into three transverse parts: the anterior head-shield or cephalon, the median thorax consisting of several movable segments, and the posterior abdominal shield or pygidium. The cephalon is generally of semicircular outline, and the posterior lateral corners, known as the genal angles, are often produced into spines. The more elevated median portion of the cephalon is the glabella, and this is separated from the cheeks or lateral portions by the usually well- marked dorsal furrows. The surface of the glabella is broken up into a series of lobes by three pairs of transverse furrows, called the lat- eral furrows. The paired lobes are called the lateral lobes and the unpaired anterior lobe, which is often very prominently developed, is known as the frontal lobe of the glabella. The cheeks are traversed in most trilobites by a sinuous line, the facial suture, along which the lateral portions or free cheeks break away from the fixed cheeks. The eyes, which in most trilobites are of reni- form shape, are situated upon the cheeks. In some species (asAcidaspis niira) the eyes are placed at the summits of high immovable peduncles; cer- tain genera, as Agnostus, Microdiscus, and Ampyx, are totally blind ; and other genera (as Trinucleus) have eye spots in their young stages and are totally blind when adult. Two types of eyes are present — compound, situated on the free cheeks, and simple eye spots, ocelli or stemmata. The compound eyes are made up of lenses, which may vary in number from li in Trimerocephalus to 15,000 in Remopleurides. The facial suture varies largely in its course across the cephalon, not only in tlie dill'erent species and genera, but also in the indiviiluals that represent different stages of development of a single spe- cies. Its position in the adult furnishes an ex- cellent diagnostic character. In most trilobites a groove, called the occipital furrow, runs near to and parallel with the posterior margin of the cephalon, and cuts off the occipital lobe from the glabella. The thorax is made up of several transverse segments which articulate with one another in such manner that some trilobites were able to roll themselves up into a ball after the manner of the armadillo. The number of segments of the thorax is usually constant in the adults of each genus, but it varies within the subclass from two in Agnostus to 29 in Harpes. The pj'gidium, or caudal shield, is a solid plate formed by the union of several segments, of which traces are still evident in the ribs of its convex dorsal sur- face. The axis and the pleural ribs are usually present, but in some species of Asaphus and Illaenus they are almost entirely suppressed. The trilobite carapace is generally ornamented by small tubercles. JIany genera have smooth or punctate tests; others have the test marked by concentric wrinkles ; and in other genera spines are developed. Ventkal Surface. As a rule the ventral sur- face of the trilobite bod}' is so closely united with the rock matrix that it is only through ex- ceptionally favorable modes of fossilization that anything at all has been learned about the nature of the ventral integument and the appendages. The principal organ on the under side of the head is a rather prominent plate, the upper lip or hypostome, which articulates with the anterior margin of the cephalon, and which protected the small mouth opening situated at its posterior end. Each segment of the carapace, excepting the hindermost anal segment, bears a pair of appendages. The foremost pair, arising near the sides of the hypostome, are simple slender whip- like antenme, much like those of the higher Crustacea. All the other appendages are bira- mous; their endopodites are six-jointed crawl- ing legs, and their exopodites are many-jointed feathery organs that probably served both for swimming and respiration. The basal joint of the cephalic limbs form jaw-like organs about the mouth. Development. Tlie ontogeny of several genera of trilobites is known. Small rounded bodies supposed to be the eggs of trilobites have been found. Those species of which continuous series of growth stages have been studied are found to originate in a minute 'protaspis' stage and to attain their adult shape through a series of pro- gressive changes expressed in the successive molts, in much the same manner as do the REPRESENTATIVE TRILOBITES. 1. Dorsa) view of a trilobite (Triarthus Becki), a species whose perfect epecimena preserved in the rtica elate (Ordovieian). near Rome. N. Y.. have .yielded most of the known facts as to the external fitructiire and appearance of these animals. 2. Ventral view of the same. 3. Cephalic shield (cephalon) of Dalmnnites If.iusmaLni iDeYnm&n): I, limb or border; mX. marjjinal furrow: ^.s.. cenal spines; gi, prlabella: f.l.. frontal lobe, beneath which are seen the 'side lobes' and 'side furrows ' of the prlabella; n.f.. occipital furrow; n.r.. occipital rinp:: f.s., facial suture: e. visual surface of e.ve; p.l, palpebral lobe. i. Median autero-posterior vertical section of a trilobite (Ceraurus pleuresan- tbeinus): c, cephalon, with h.ypostoma below it; p, test of the abdomen (p.vpidiuni): rn, month; v.m.. ventral mem- brane; i.e.. intestinal canal. 5. A trilobite (Phacops latifrovn: Devonian), rolled up; f^l. plabella: e. eye. 6. Develop- ment (ontoffen.v) of a trilobite (Sao hirsuta: f'anibriant; 1, Ist larval stasre (protaspis): 2-0. successive 'nepionic' stapes of development 7. Ampys nasuta (Ordovieian). 8. Deiphnn Forbpsi (Silurian). 9. Proetiia Bnhemirus (Silurian). 10. /far/)efi j/n^w/a (Ordovieian ). 11. TrJDCuleus Gnldfi3ssH0Tiovci&n). 12. Profile view of . Sy^/jare.voc/jH.s- Tu/ru.'! (.Silurian). 13 Cbeinirus insi^nis (Silurian). U. Megalaspis estenuata (Ordovieian). 15. CoDocoryphe Sulzeri (Cambrian) without the free 'cheeks.' (After Zittel.)