152 



INTRODUCTION TO EVOLUTION 



swimming bel 



reproductive 

 orgoins 



branch (rhoibdosome) 



with motny polyps 



FIG. 8.6. Graptolite colony. (After Bossier.) 



cephalopods the suture lines were smooth curves (Fig. 8.8). In later pe- 

 riods cephalopods having suture lines following complex configurations 

 were found, as noted below. Cephalopods having smoothly curved suture 

 lines are called nautiloids. Interestingly enough, both our modern 

 nautili and these earliest cephalopods were of this type. The principal dif- 

 ference between the shells of Ordovician nautiloids and those of their 

 modern descendants relates to the coiling of the shell. Some of the Ordovi- 

 cian cephalopods had straight shells (Fig. 8.9), others were loosely coiled, 

 while still others were closely coiled like those of modern nautili. In Fig. 

 8.9 the smooth sutures are visible in places where the outer surface of the 

 shell has been chipped away. Cephalopods included the largest animals 

 living in Ordovician seas; some of the straight-shelled forms reached a 

 length of 15 feet. 



Many other invertebrates were present in the Ordovician. Among them 

 were the bryozoans, lowly animals which have constituted an important 

 agent of limestone formation in oceans ever since their first appearance. 

 Among the other invertebrates we should not forget the trilobites, which 

 as in the Cambrian formed an important constituent of the fauna, the great 

 variety of forms suggesting adaptations to many conditions of aquatic life. 



