ST. HILDA'S SNAKE-STONES. 105 



chambers are constructed one after the other as the 

 animal's growth demands more room. The deserted 

 chambers are then walled off by a curved pearly 

 division or septum, the communication between the 

 chambers being, however, kept up by means of a 

 membranous tube, called the siphuncle (i), which 

 is continued throughout the whorls of the shell. 



FIG. 20. Section of shell of Ammonite. 



In the Nautilus this siphuncle runs through the 

 central part of the chambers, whereas in Ammonites 

 it lies on the outside or dorsal portion. The creature 

 has a heart, four pyramid-shaped branchiae or gills, 

 two large eyes attached by short stalks to the sides 

 of the head, with various organs concerned in repro- 

 duction. Its tentacles are prehensile, and are useful 

 in procuring food, two of them being fused together 



