26o AN INTRODUCTION TO THE STUDY OF FOSSILS 



mouth and a funnel-shaped swimming organ, the hyponome. 

 The mouth is provided with jaws and a radula. The nervous 

 system is highly developed. 



A shell may be present or absent and when present may be 

 external or internal, and modified into various forms. The 

 general method of shell secretion in the moUusks is described 

 under the Pelecypoda, pages 214-216, 220. 



If oriented with the head and arms down, in the position 

 assumed by Nautilus when moving over the sea bottom, the 

 visceral mass is on top as in the normal position of gastropods, 

 but unlike gastropods is not twisted. The shell likewise differs 

 in being coiled dorsally, i.e. the outside of the curved shell of 

 Nautilus is the ventral portion, of gastropods the dorsal. (Com- 

 pare Figs. 113 and no.) 



Cephalopods are exclusively marine. The sexes are separate. 



Derivation of name. — > Greek cephale, head, -\- pons {pod), 

 foot. The foot of other mollusks is here transformed into the 

 arms and funnel about the head. 



Cephalopods are divided on the basis of the number of gills, 

 into two orders. 



1. Tetrabranchiata. 



2. Dibranchiata. 



Order i, Tetrabranchiata 



Cephalopods possessing an external shell of many chambers, 

 only the last of which is occupied by the body of the animal. 

 Head region bearing many appendages with suckers, and saucer- 

 shaped eyes open to the exterior and without lenses. Four 

 gills and four kidneys present, but no ink sac. 



Named in reference to the number of gills from Greek tetra, 

 four, + hranchia, gills. 



This order is represented by but one Hving genus, Nautilus, 

 which has flourished from Tertiary to present times. There 

 are, however, a great many fossil cephalopods found in Paleozoic 

 and Mesozoic rocks which, though not affording much informa- 



