258 AN INTRODUCTION TO THE STUDY OF FOSSILS 



retina is bathed by sea water on one surface and receives the 

 fibers of the optic nerve on the other. 



It is considered probable from experiments in baiting Nautilus 

 that the animal is guided in the capture of its prey largely by the 

 sense of smell ; though the eyes are doubtless likewise useful, 

 still their simple character and the darkness of the depths where 

 Nautilus is usually found would render them less useful guides. 



The organs of hearing or of balancing are the two otocysts, 

 — little sacs near the nerve collar, which are almost completely 

 filled by a large number of otoliths, — tiny crystals of calcium 

 carbonate. 



Four modified tentacles, the so-called ocular tentacles, are 

 supposed to serve an olfactory function. They differ from the 

 other tentacles in the possession of vibratile cilia, and their 

 sensitiveness is shown by their instant retraction when touched 

 by a foreign body, — a sensitiveness not shared by the other 

 tentacles, whose function is mainly adhesive. (The Dibranchiata 

 do not possess these accessory olfactory tentacles, and it has been 

 suggested that this difference is to be correlated with the fact 

 that while Nautilus finds its food chiefly by the sense of smell, 

 the Dibranchiata find it chiefly by the aid of their remarkably 

 perfect eyes.) 



The four osphradia situated in the mantle cavity near the gills 

 are probably sense organs ; they are knob-like projections con- 

 taining nerve cells. 



The sexes are separate, each individual possessing a single 

 testis or ovary near the posterior end of the body. These dis- 

 charge their products through ducts into the mantle cavity. 



The eggs are laid singly, each egg being about an inch and a 

 half long and surrounded by two thick shells. 



The development of Nautilus is unknown. 



1. What is the present habitat of Nautilus? Its geologic 

 range ? 



2. Sketch the shell of a living species of Nautilus (a) side 

 view; (b) view looking into aperture ; (c) cut portion of section 



