ALLIES OF THE ACEPHALA 



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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 

 their shells showed coiling more and 

 more to the close coil of the present-day 

 nautilus. All the hundreds of species of 

 nautiloids, except four species of the 

 nautilus, disappeared as living things 

 ages before man came into existence 

 upon the earth. 



Definition of Cephalopoda (Gr. kephale, 

 "head"; pous (pod-), "foot"). Because 

 of their structural relationship, the 

 squid, octopus, nautilus, and the fossil 

 nautilus-like forms belong in a class to- 

 gether, the Cephalop'oda. 



In this class the body has a distinct 

 head. No part shows indications of be- 

 ing divided into somites. The head has 

 two large eyes. The mouth is surrounded 

 by divisions of the primitive foot, called 

 arms or tentacles. These divisions of 

 the foot either have sucking disks for 

 holding on, or smooth surfaces which 

 perform the same function. In the 

 mouth there is a parrot-like beak and a 

 rasping tongue. A shell-forming mantle 

 either incloses the shell, as in the squid, 

 or lies beneath it, as in the nautilus and 

 in Orthoceras. 



There are two gills in the mantle cavity in the squid, and 

 four in the nautilus. A siphon is present in all the exam- 

 ples described. The sexes are separate. 



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Fig. 118. Fossil Or- 

 thoceras. (Reduced) 



After Blake 



