MOLLUSCA — CEPHALOPODS 



271 



Light rays passing through the protec- 

 tive transparent cornea are bent towards 

 the pupil by the water iilUng the external 

 chamber of the eye ; this chamber is open 

 by a small hole to the sea water. The 

 pupil is an opening through the firm wall 

 of the eye ; the portion of this wall im- 

 mediately bounding the pupil and called 

 the iris contains muscle libers by whose 

 action the pupil can, to a limited extent, 

 be enlarged or diminished. The light is 

 then focused upon the retina by passing 

 through the spherical, dense lens and 

 next through the large mass of jelly-like 

 vitreous humor. 



There are two otocysts in the posterior 

 portion of the head ; these have the power 

 of coordinating movements in maintain- 

 ing equilibrium, and are possibly also 

 functional in hearing. There may also 

 be present a sense of smell and of taste. 



The sexes are distinct. The eggs are 

 fertilized through copulation. 



Belemnites (Fig. 122). 



Jurassic to Cretaceous. 

 Belemnites is closely related to the 

 squid. Impressions of the tentacles and 

 arms are sometimes found preserved in 

 the fossil state, showing the suckers pro- 

 vided with horny hooks. The mandibles 

 and ink sac are likewise at times found 

 >with the ink preserved in the form of solid 

 carbon particles. The shell, however, is 

 more complete than that of the squid, 

 consisting of three parts, — guard, phrag- 

 mocone, and proostracum. The guard is 



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