22 RELICS OF PRIMEVAL LIFE 



have any certain evidence is that of the Crustacea, 

 the group to which our modern lobsters and crabs 

 belong, and its most prominent representatives are 

 the trilobites (Figs. I, 2), so called from the three 

 lobes into which the body is divided. These creatures 

 are indeed remarkable for the twofold property of 

 bilateral symmetry, and fore and aft jointed structure, 

 both based on the number three. From front to 

 rear we have a large head, usually with well-devel- 

 oped eyes and oral organs, a middle or thoracic 

 part composed of a series of movable segments, and 

 a tail-piece sometimes small, sometimes nearly as 

 large as the head. Transversely, the body is divided 

 into a central and two lateral lobes, which can be 

 seen in the head, the thorax, and usually in the 

 tail as well. The organization of these animals 

 must have been as complex as that of most existing 

 Crustaceans. Their nerve system must have been 

 well developed ; a vast number of muscles were 

 required to move the different parts of the trunk, 

 and the numerous and complex limbs which have 

 been observed in some of the species, and no doubt 

 were possessed by all. Their digestive and circu- 

 latory organs must have been in proportion to the 

 complexity of their locomotive organs. Figure 2, 



