18 



Guide to the Invertehrata. 



Nautilus the fore foot, which surround the mouth, carry numerous sheathed 

 GALLEEY tentacles (not suckers). There are two pairs of comh-like gills, 

 and two pairs of excretory organs. There are two oviducts, right 

 la, and left, in the female, and two sperm-ducts in the male, the left 



duct in both being rudimentary. The eyes are hollow chambers 

 opening to the exterior by minute orifices (pin-hole camera) and 

 devoid of refractive structures. There is no ink-sac present in 

 this order. (!See Figs. 18, 19, 37, and 38.) 



Fig. 37. — Animal and shell of living Nautilus pompilius, Linn. (The shell is 

 cut open to show the deserted hinder chambers.) 



Fig. 38. — Section of shell of Nautilus showing septa and empty body- chamber. 



A strong and well-developed external shell, either coiled or 

 straight, is present, and is not enclosed in a fold of the mantle 

 (except perhaps in such narrow-mouthed shells as Gomphoceras, 

 which was probably enclosed in the mantle, as is that of Spirula 

 amongst the Dibranchiata). The shell consists of a series of 



