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TRILOBITES. 
fresh and salt water, and their organisation seems to show that they were powerful 
swimmers; considering, too, the large size which some of the species attained, 
examples of Pterygotus reaching a length of from 4 to 6 feet, there is little 
doubt that these monstrous sea-scorpions were the masters of the ocean in 
Palaeozoic times. A third order is represented by the extinct Trilo- 
Trilobites. . . 1 J 
bites or Trilobata, which swarmed in the seas of the Palaeozoic epoch, 
and are amongst the earliest of known fossils. The name Trilobite, or three- 
lobed, is given to them because in the best known and typical members of the 
group the body is divisible into three distinct parts—an anterior cephalic shield 
corresponding to the head of the Crustacea and to the cephalothorax of Limulus, 
and formed, as in Crustaceans, of five fused segments; a median thoracic portion, 
composed of a variable number of freely movable 
segments; and the pygidium, also composed of a 
variable number of segments, but usually fused to 
form a great caudal shield. The lateral portions 
of the segments are produced sideways into great 
pleural plates, which mostly conceal the limbs, 
and the hinder angles of the cephalic shield are 
frequently prolonged into sharp spiniform processes, 
sometimes so long that they project backwards 
beyond the hinder end of the body. On the upper 
side of the cephalic shield there are a pair of large 
kidney-shaped compound eyes, but no sign of the 
simple eyes present in the Xiphosura and Meros- 
tomata has been discovered. For many years no 
trace of limbs could be detected, but it is now 
known that a pair of limbs was attached to the 
lower surface of each of the segments of the head 
and body; though instead of there being two pairs 
situated in front of the mouth, as in Crustaceans, 
there was only one, as in the Xiphosura, Merostomata, 
and Arachnida. These, however, take the form of 
long filiform antennae, and are placed on each side 
of a large upper lip, or labrum, behind which comes 
the mouth. The rest of the appendages of the head, as well as those of the 
thorax, are alike, consisting of a large basal segment, from which spring two 
branches, an inner, which was used for crawling, and an outer, many-jointed and 
fringed with bristles, which was perhaps used for swimming. The basal segments 
of these limbs in the head region were utilised as jaws, and in the pygidium the 
inner branches, or endopodites, were flattened and more or less leaf-like as in the 
lower Crustaceans, such as Apus. There is little doubt that Trilobites, instead 
of swimming in the open sea and leading an active predatory life, spent their time 
crawling or swimming slowly along the bottom, feeding upon worms, burrowing 
in the mud, and, in case of danger, rolling up tightly into a ball like wood-lice. 
Many specimens are found fossilised in this condition, with the lower surface of 
the pygidium pressed against the head. 
a trilobite ( Triarthrus )= 
(From Beecher.) 
