26 RELICS OF PRIMEVAL LIFE 



the bristle - footed marine worms ; but there is a 

 vast gap, both in the Cambrian and Modern seas, 

 between any of these worms and the Crustacea, 

 which, either as embryos or as adults, have any re- 

 semblance to them. 



The Trilobites, after appearing in a great variety 

 of generic and specific forms, and playing a most 

 important part in their time, were not destined to 

 continue beyond the Carboniferous period, and be- 

 fore that time they were beginning to give place to 

 the Limuli, King-crabs, or Horseshoe-crabs, a few 

 species of which continue on our coasts until the 

 present time. In this limited duration the Trilobites 

 present a strange contrast to certain shrimp - like 

 Crustaceans, their contemporaries (the Phyllopods), 

 which very closely resemble some still extant, and 

 the same remark applies to swarms of little bivalve 

 Crustaceans (Ostracods), which are still represented 

 by hosts of modern species both in the sea and in 

 the fresh waters. There is, however, a remarkable 

 group of shrimp-like Crustaceans, represented in the 

 modern world by only a few small species, which in 

 the Cambrian age attained greater size, and consti- 

 tute a very generalized type combining characters 

 now found in lower and higher groups of Crustacea. 



