CEPHALOPODA. 



215 



foui' in number, two on each side of the body ; the funnel does not 

 form a complete tube ; and there is no ink-bag. 



Though abundantly represented by many and varied fossil forms, 

 the only living niemljer of the Tetrahranchiata with which we are 

 acquainted is the Pearly Nautilus, which has long been known by 

 its beautiful chambered shell. The shell of the Pearly Nautilus 

 (fig. 155) is coiled into a spiral, and is many-chambered, the cham- 

 bei's being walled off from one another by curved shelly partitions or 

 septa, perforated centrally by a foramen which transmits a mem- 



Fi^^ l'»:i. — Cephalopoda. Sepia dcgans. 



y\^. 154. — Anatomy of 5pirw?tt 



('»s/7'f(?/,s(atter Owen), show- 

 ing the po.sition of llie 

 skeleton. 



branous tube or siphuncle. The animal inhabits only the last and 

 largest chamber of the shell, from which it can protrude its head at 

 will. The function of the chambers of the .shell is n(it very clearly 

 understood ; but it ajjpears to be that of retlui/ing the specific 

 gravity of the shell t(j near that (jf the suiroundiiig water; since 

 they appear to be filleil with some giis ajiparently secreted by the 

 animal. Good authorities, however, believe that these chambers 

 are filled with water. The siphuncle does not connnunicate in any 

 way with the chandlers of the shell, and its functions are also un- 



