260 MOLLUSCA. CEPHALOPODA 



ORDER I. TETRABRANCHIA 



The Tetrabranchia have four gills, and an external 

 chambered shell. There are two Sub-Orders, (1) Nauti- 

 loidea, (2) Ammonoidea. 



SUB-ORDER I. NAUTILOIDEA 



In Palaeozoic times the Nautiloid Cephalopods were 

 very abundant, but at the present day the only repre- 

 sentative of the group is Nautilus. This possesses two 

 pairs of gills, and two pairs of auricles ; no ink-sac is 

 present; and the funnel is not a complete tube, but is 



-d 



Fig. 103. Section of the shell of Nautilus pompilius, Recent, a, body- 

 chamber ; b, septum; c, septal neck; d, siphuncle. x ^. 



formed of two parts. Around the mouth are numerous 

 lobe-like processes which are given off from the margin of 

 the head ; these do not bear suckers, as is the case in the 

 Dibranchs, but tentacles which can be retracted within 

 sheaths. The jaws are calcined and are not uncommonly 

 found fossil. 



