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TRILOBITES. 



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- 

 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.) 



