LIFE IN THE PALEOZOIC EPOCH 



461 



The curious Trilobites (Trilobita, fig. 1315), which constituted 

 a closely allied class, were dominant in the older Palaeozoic periods, 

 but became entirely extinct before the end of the epoch. In these 

 creatures the upper side of the body was covered by a firm in- 

 vestment divided into a head-shield, a varying number of thoracic 

 segments, and a tail-shield. There was also, as a rule, a longi- 

 tudinal division into three regions, 

 and this is the origin of the name 

 " trilobite ". The numerous species 

 exhibited a great range of characters, 

 both as regards size, shape, and other 

 features. The upper side of the head- 

 shield often bore a pair of eyes, fre- 

 quently large and facetted, but visual 

 organs were sometimes entirely ab- 

 sent. Our knowledge of the under 

 surface and limbs of trilobites was very 

 incomplete till comparatively recently, 

 partly on account of the delicacy of 

 these parts, but now, chiefly owing 

 to the investigations of American 

 geologists on certain well-preserved 

 species, many points relating to them 

 have been elucidated. The head 

 carried a pair of slender feelers, and 

 there were numerous pairs of forked 

 limbs used for crawling and swim- 

 ming, while some of those in the 

 region of the mouth acted as jaws 

 (fig. 1316). Many of the stages in 

 growth have been observed, and it may be said that trilobites 

 which, when adult, are of simple structure, resemble the early 

 stages of those which attain to greater complexity. This is 

 precisely what the doctrine of evolution would lead us to expect. 

 It may be added that many trilobites were able to roll up like 

 hedgehogs, and this was no doubt a means of protection. 



Appearing rather later in time than the Trilobites, which they 

 to some extent supplanted, we find Eurypterids (e.g. Pterygotus, 

 fig. 1317), some of which attained a length of about 5 feet. 

 They died out before the end of the epoch, and appear to have 



Fig. 1316. A, Under side of a Trilobite (Tri- 

 arthrus] restored, showing the numerous jointed 

 limbs, ant., antennae; ./., upper lip; /./., lower 

 lip. B, Diagrammatic cross-section through same, 

 showing limb-regions, j, Projection serving as a 

 jaw; sw. and cr., swimming and creeping branches. 



