20 THE ORIGIN OF MAN 



bivorous, sometimes carnivorous and scavengers. 

 The carnivorous forms known as drills bore holes 

 into their shelled victims, with the radula and the 

 aid of a weak secretion of sulphuric acid. The 

 head of most gastropods is usually distinctly 

 marked off from the rest of the body, and has a 

 pair of eyes and one or two pairs of sensory or- 

 gans or tentacles. Cephalopoda means head- 

 footed and is the most highly organized class of 

 the molluscs and includes such animals as the 

 nautilus, ammonites, octopus, cuttlefish and 

 squid. They all are exclusively marine. 



Having concluded the first period of the Early 

 Paleozoic era, we shall then pass into the second 

 or Ordovician period which comprises the rise of 

 nautilids, armored fishes, land plants and corals. 



In the Lower Ordovician the first true grapto- 

 lites existed, also many gastropods, some of which 

 were large and thick-shelled. There was also a 

 variety of straight, bent and coiled nautilus, and 

 many trilobites. During the middle Ordovician 



