THE KING CRABS 



3219 



Trilobites 



swimmers; considering, too, the large size which some of the species attained, ex- 

 amples of Pterygotus reaching a length of from four to six feet, there is little doubt 

 that these monstrous sea scorpions were the masters of the ocean in Paleozoic 

 times. 



A third order is represented by the extinct Trilobites or Trilobata, 

 which swarmed in the seas of the Paleozoic epoch, and are among 

 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 and 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 fre- 

 quently prolonged into sharp spiniform processes, 

 sometimes so long that they project backward be^ 

 yond 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 Merostomata 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 antennse, 

 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 utilized 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 fossilized in this condition, with the 

 lower surface of the pygidium pressed against the head. 



A TRILOBITE (Triarthrus). 

 (From Beecher.) 



