With those considerations in mind, the claim of the extinct group of the trilobites to be considered as representatives of the lower and more primitive steps in the Arachnidan genealogy must, it seems, receive a favourable judgment.
But most important of the evidences presented by the trilobites of affinity with Limulus, and therefore with the Arachnida, is the tendency less marked in some, strongly carried out in others, to form a pygidial or telsonic shield - a fusion of the posterior somites of the body, which is precisely identical in character with the metasomatic carapace of Limulus.
38), like that of Limulus and Scorpio, and that lateral spines on the pleura of the somites are frequent as in Limulus, and that neither metasomatic fusion of somites nor post-anal spine, nor lateral pleural spines are found in any Crustacean, nor all three together in any Arthropod besides the trilobites and Limulus - the claim of the trilobites to be considered as representing one order of a lower grade of Arachnida, comparable to the grade Entomostraca of the Crustacea, seems to be established.
The fact that the single pair of prae-oral appendages of trilobites, known only as yet in one genus, is in that particular case a pair of uni-ramose antennae - does not render the association of trilobites and Arachnids improbable.
Thus, then, it seems that we have in the trilobites the representatives of the lower phases of the Arachnidan pedigree.
On the 1 A pair of round tubercles on the labrum (camerostome or hypostoma) of several species of Trilobites has been described and held to be a pair of eyes (22).
That any partial fusion of originally distinct chitinous plates takes place in the cephalic shield of Trilobites, comparable to the partial fusion of bony pieces by suture in Vertebrata, is a suggestion contrary to fact.
The Trilobites are known only as fossils, mostly Silurian and prae-Silurian; a few are found in Carboniferous and Permian strata.
The posterior carapace of the Trilobites and of Limulus is probably enough in origin a telsonic carapace - that is to say, is the tergum of the last segment of the body which carries the anus.
For the literature of Trilobites see ('22*).
Lindstrom, G., " Researches on the Visual Organs of the Trilobites," K.
Trilobites have been discovered in Rikuzen.
TRILOBITES, extinct Arthropoda, formerly classified with the Crustacea, but of late years relegated to the Arachnida, which occurred abundantly in seas of the Cambrian and Silurian periods, but disappeared entirely at the close of the Palaeozoic epoch.
However different in structure Trilobites may be, they all agree in possessing a head-shield usually semi-circular in shape, which results from the fusion of apparently five segments, and bears, except in some blind forms, a pair of large reniform compound eyes like those of the king-crab (Xiphosura).
It is with respect to this number of segments that respectively constitute the pygidium and the midregion of the body that Trilobites differ most markedly from each other; and it is a singular fact that the extremes in structural organization in this particular to be met with in the Trilobita are found side by side in strata of Cambrian age.
For many years only the dorsal surface of Trilobites was known, nothing having been ascertained of the ventral surface and appendages.
Such was the structure of the appendages in Trilobites belonging to the genus Triarthrus; but considering the great structural differences that obtain between Triarthrus and many other genera, it would be rash to assume that there were not corresponding differences in the structure of the limbs.
It is probable that no satisfactory classification of the Trilobites will be proposed until the limbs of most of the genera have been examined.
In the majority of Trilobites this groove passes backwards from the anterior or anterolateral edge of this plate to its posterior or postero-lateral border, dividing it into an inner portion continuous with the flabellum and fused tergal regions, and an outer portion bearing the eye.
In external form Trilobites are not unlike Isopod Crustaceans, especially the terrestrial species commonly called "woodlice"; and until the nature of their appendages was known, it was thought by some authorities that the two groups might be related.
As in other parts of the world, the system here contains abundant fossils, among which trilobites, brachiopods and worms are the most abundant.
Graptolites had declined notably as compared with the Ordovician, and the trilobites passed their climax before the end of the period.
Among the more important features of the marine life of the period were (1) the great development of the molluscs, especially of cephalopods; (2) theabundanceoflargebrachiopods; (3) theaberrant tendencies of the trilobites; (4) the profusion of corals; and (5) the abundance, size and peculiar forms of the fishes.
Trilobites during this period sank to a very subordinate position, but Ostracods (Cythere, Kirkbya, Beyrichia) were abundant.
Balmae, on the southern shore of Kirkcudbrightshire, the coast south of Girvan and the limestone quarries of the Stinchar and Girvan valleys, in Ayrshire, for shells, trilobites, corals, &c.
In the Cambrian period trilobites had already attained their maximum size; some species of Paradoxides were nearly 2 ft.
Many of the Cambrian trilobites appear to have been blind, and they had not at this period developed that flexibility in the carapace that some forms acquired later.
No Cambrian rocks are such as would be formed in the abysses of the sea - although the absence of well-developed eyes in the trilobites has led some to assume that this condition was an indication that the creatures lived in abyssal depths.
The upper divisions consist of bituminous limestones, clay-slates, alum-slate, and contain numerous species of trilobites of the genera Paradoxides, Conocoryphe, Agnostus, Sphaerophthalmus, Peltura, &c. The Ordovician formation occurs in two distinct facies - the one shaley and containing graptolites; the other calcareous, with brachiopods, trilobites, &c. The most constant of the calcareous divisions is the Orthoceras limestone, a red or grey limestone with Megalaspis and Orthoceras.
The typical Silurian rocks are richly fossiliferous, the shales containing trilobites, the sandstones many brachiopods, and the limestones a rich coral and bryozoan fauna.
A character of great diagnostic value in the more primitive Arachnida is the tendency of the chitinous investment of the tergal surface of the telson to unite during growth with that of the free somites in front of it, so as to form a pygidial shield or posterior carapace, often comprising as many as fifteen somites (Trilobites, Limulus).
Our general conclusion from a survey of the Arthropoda amounts to this, that whilst Peripatus, the Diplopoda, and the Arachnida represent terrestrial offshoots from successive lower grades of primitive aquatic Arthropoda which are extinct, the Crustacea alone present a fairly full series of representatives leading upwards from unspecialized forms. The latter were not very far removed from the aquatic ancestors (Trilobites) of the Arachnida, but differed essentially from them by the higher specialization of the head.