CEPHALOPODA. 



215 



four 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 member of the Tetrabranchiata 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 

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

 bers being walled off from one another by curved shelly partitions or 

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



Fig. 153. — Cephalopoda. Sepid elegans. 



Fig. 154. — Anatomy of 5jnru?a 

 australis (sifter Owen), show- 

 ing the position of the 

 slteleton. 



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 not very clearly 

 understood; but it appears to be that of reducing the specific 

 gravity of the shell to near that of the surrounding water; since 

 they appear to be filled with some gas apparently secreted by the 

 animal. Good authorities, however, believe that these chambers 

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

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



