212 INVERTEBRATE ANIMALS. 
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 Zetrabranchiata 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. 113) is coiled into a spiral, and is many- 
chambered, the chambers being walled off from one another 
by curved shelly partitions or septa, perforated centrally by 
a foramen which transmits a membranous tube or siphuncle. 
The animal inhabits only the last and largest chamber of the 
Fig. 113-—The Pearly Nautilus (Vautilus pompilius), a Mantle ; 
6 Its dorsal fold; ¢ Hood; o Eye; ¢ Tentacles ; J Funnel. 
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 
unknown, except that it must certainly serve to maintain the 
vitality of the shell. 
