252 AN INTRODUCTION TO THE STUDY OF FOSSILS 



The function of the siphon is unknown. Some have supposed 

 it to be a hydrostatic organ, assisting the animal to rise and 

 sink through the water. And some have considered that what- 



FiG. 112. — Nautihis pompilius, from the Philippine Islands. (X5.) The animal 

 is seen from the right side, the right half of the shell having been cut away, an.m., 

 annular muscle which holds the body of the animal in the shell ; the posterior 

 portion of the muscle is attached at the junction of the last septum with the shell ; 

 d.m., dorsal portion of mantle ; e., eye ; /., funnel ; ho., hood, which, like the oper- 

 culum of gastropods, may be drawn in against the aperture to close the shell ; 

 0'., 0" ., anterior and posterior ocular tentacles ; si., siphon, the fleshy prolongation 

 of the mantle extending backward through the empty chambers; si., siphuncle, 

 the shelly covering of the siphon ; sp., septum, one of the transverse partitions 

 of the shell; /e., tentacles ; i^.w., ventral portion of mantle; y., earliest formed por- 

 tion of the calcareous shell. (Redrawn from GriflSn.) 



ever be its function it is in Nautilus a vestigial structure which 

 has become progressively reduced during the course of evolu- 

 tion of the Nautiloidea. 



When creeping on the sea bottom the tentacles surrounding 

 the mouth are applied to the surface over which the animal 

 is traveling and movement is effected by them (Fig. 113). 



