48 



Guide to the Mollusca. 



Order 1 TETRABRANCHIA. 



Nautilus, of which four species arc known from the Indo- 

 Pacific, differs from all other living Cephalopoda in having two 

 pairs of gills instead of one pair, and a large number of small 

 retractile tentacles without suckers instead of eightorten long arms. 



The shell is external, coiled, and formed of many compartments, of 



Fig. 42. 



-._—: ■•- .-. „ 



The Pearly Nautilus (Nautilus pompilius). 



a. body : b. siphuncle ; c. eye ; d. hood ; e. tentacles ; /. muscle 



of attachment to the shell. 



Fig. 43. 



A. Complete specimen. 



Shell of Spirvla peronii. 

 B. Section of shell showing septa (a) and Biphuncle ('V 



which only the last is occupied by the animal, although the septa 

 between the compartments are traversed by a shelly tube con- 

 taining a prolongation of the tissues of the mantle. Nautilus 

 sometimes swims at the surface, but more often crawls about at 

 the bottom in search of small Crabs, molluscs, &c, on which it 

 feeds. According to Dr. Willey, who studied its habits in the 

 Western Pacific, it is gregarious and nocturnal in its activities. 

 The fossils known as Ammonites are shells similar in structure 

 to that of Nautilus, and they are placed in this order. 



