MOLLUSCA. CEPHALOPODA. 171 



ORDER. TETRABRANCHIATA. 



The only genus of the Tetrabranchiata existing at the 

 present day is Nautilus, but in past geological times the 

 order was represented by very numerous and varied forms, 

 such as Orthoceras, Nautilus, and Ammonites. The Tetra- 

 branchs are characterised by possessing two pairs of gills, 

 two pairs of auricles, and two pairs of kidneys. An ink- 

 sac is absent ; the funnel is not a complete tube, but is 

 formed by the apposition of two troughs. The arms are 

 numerous and do not bear sucking-discs. 



A shell is present in all forms and is always external. 

 It consists of a tube, which tapers to a point at one end, 

 and may be straight, arched, or spiral ; in the spiral forms 

 the whorls may be separate, or partly free, or in contact 

 throughout ; in some cases they are placed all in one plane, 

 in others arranged obliquely. The cavity of the shell, 

 unlike that in the gasteropods, is not continuous through- 

 out from the aperture to the apex, but is divided up into 



FIG. 49. Section of shell of Nautilus, a, body-chamber ; b, septum ; 

 c, septal neck ; d, siphuncle. 



