PRE-CAMBRIAN AND PALEOZOIC ERAS 



151 



polyps wi+h 



tentacles 



extended 





^^' 



V;: 



FIG. 8.5. Coral. A, coral animals (polyps) seen from above, and, at left, the empty 

 cups remaining after their death. B, coral polyps of a species suggestive of Hydra, a 

 free-living coelenterate (Fig. 4.15, p. 71). 



lilies look not unlike flowers growing in the ocean (Fig. 8.7). The stem or 

 stalk attaching the creature to the ocean floor is composed of piled rings 

 fastened together. These rings, freed by disintegration of crinoid stems, are 

 immensely abundant in many Ordovician rocks. 



Brachiopods continued abundant throughout the Ordovician. Snails, 

 relatively rare in the Cambrian, became abundant in Ordovician oceans. 

 Among the molluscs the greatest prophecy of things to come was pre- 

 sented by the occurrence of cephalopods. 



Cephalopods are the group of molluscs to which squids, octopi, and 

 nautili belong. The chambered nautilus (Fig. 8.8) of our modern oceans 

 presents a structure not unlike that of its Ordovician ancestors. The animal 

 itself is soft-bodied and unsegmented; it possesses a pair of eyes and a 

 cluster of extensible, sucker-bearing arms or tentacles around the mouth. 

 As shown in the figure, the animal lives in the outermost compartment of 

 its tapered shell. When it grows it moves outward, adding to its shell and 

 secreting behind it a wall or septum. Thus the shell eventually consists of a 

 series of chambers or compartments, evidence of successive stages in the 

 growth of the animal (Fig. 8.8). Where each septum joins the side wall of 

 the shell a line of attachment, called a suture, is formed. In the earliest 



