MOLLUSCS AND CRUSTACEANS. 59 



perceive that among the first traces of animal 

 life, in the remotest periods of the natural his- 

 tory of our planet, there were many species 

 which still continue to exist. There were also 

 numerous simple univalve mollusca, or Gastero- 

 poda, such as Euomphalus; and many Ceph- 

 alopoda, such as Orthoceras and Actinoceras. 

 Various examples of Annelida, or animals whose 

 bodies consist of articulated segments or rings 

 likewise occur in the Silurian strata, some of them 

 being naked marine worms, and others those which 

 are covered with shells, of which the Nereis, the 

 Grordius, the Serpula, are instances. Of all these, 

 distinct imprints have been discovered in the 

 limestone rocks of the period referred to. 



The CRUSTACEA of the Silurian system include 

 a very remarkable and peculiar race of creatures 

 which are called Trilobites, from thfc circum- 

 stance of their bodies consisting of three lobes 

 or divisions. Of these creatures there are no living 

 representatives: they are entirely restricted to 

 the Palaeozoic or most ancient fossiliferous de- 

 posits. In these remarkable animals the body is 

 protected by -a strong case or shell, composed of 

 numerous ring-shaped segments or arches, and 

 as already stated is divided into three lobes by 

 two longitudinal furrows. The head and the 

 abdomen are, however, enclosed in a single piece 

 of armour. The eyes are large and are found to 

 consist of numerous facets or lenses, and are thus 

 similar in structure to the eyes of several kinds 

 of crustaceans at present existing, as well as those 

 of various insects, such as the house-fly and the 



