ALLIES OF THE PELECYPODA : MOLLUSCA 193 



Fossil Relatives of Nautilus. Many millions of years ago, in 

 the early history of the earth, the most ancient of the imme- 

 diate ancestors of Nautilus lived. Paleon- 

 tologists (students of fossils) have given its 

 fossil (Fig. 98) the name Orthoc'eras (Gr. 

 orthos, straight; keras, horn). The strongest 

 evidence we have that Orthoceras is a rela- 

 tive of Nautilus is the series of chambers 

 joined by the siphuncle. We know nothing 

 of the structure of the soft parts of Orthoc- 

 eras, but paleontologists have made draw- 

 ings to show how Orthoceras probably 

 appeared. From the study of fossils in 

 later layers of the earth's crust we know 

 that the group of nautiloids (Nautilus-like 

 animals) grew to be of large size, and to 

 have their shells coiled more and more to 

 the close coil of the present-day Nautilus. 

 All of the hundreds of species of nautiloids, 

 except four species of Nautilus, disap- 

 peared as living things ages before man 

 came into existence upon the earth. 



Definition of Cephalopoda (Gr. kephale, 

 head ; pous (pod), foot). Ommastrephes, 

 Nautilus, and Orthoceras, because of their 

 structural relationship, belong in a class 

 together, the Ceplialop'oda. 



In this class the body has a distinct head. 

 No part shows indications of being divided 

 into somites. The head has two large eyes. 



The mouth is surrounded by divisions of FIG. 98. Fossil Orthoc- 

 . P , eras. Reduced, 



the primitive foot, called arms or tentacles. (After Blake) 



These divisions of the foot either have 

 sucking-disks for holding on, or smooth surfaces which per- 

 form the same function. In the mouth there is a parrot-like 



