THE EYES OF MOLLUSCS 153 



influence in bringing about the gradual extinction of these two Orders. 

 The Nautilidae and Ammonites are a side-branch of the class Mollusca 

 which has almost completely disappeared, but the preservation of the 

 simple type of eye found in Nautilus, which has neither lens nor any 

 power of independent movement of the eyeball, well illustrates the way in 

 which an organ which appears to be of subsidiary importance to the life of 



Fig. 113. 

 Drawing taken from photograph of a living specimen of Nautilus pompihus, 

 showing the position of the eye when the animal is in the water. (After A. 

 Willey, Q. J. Micro. Sc. 39.) Note. — The groove which runs vertically 

 downwards from the central aperture to the lower margin of the eye. 



the animal as a whole, may be retained as an hereditary structure through 

 millions of years, without — it may be presumed — having undergone any 

 marked change either in the way of evolution or devolution. 



A similar preservation of a simple type of eye, adapted to simple 

 needs, has presumably occurred in other types of shellfish belonging to 

 the Class Mollusca and throughout the Invertebrate Kingdom, in all the 

 simpler and less specialized types of animals in which eyes have been 

 evolved ; and it may be inferred from the similarity of the shells of extinct 

 animals and those of the present day that the adult and larval forms of the 

 extinct species correspond with the adult and larval forms of the living, not 

 only in general but also in detail — for example, the structure and relative 

 position of the ocelli and statocysts of the veliger larvae of the extinct parent 

 stock with those of the living species ; moreover, when we compare the 

 trochophore and veliger larvae of the Mollusca with those of other classes of 



