152 



THE PINEAL ORGAN 



the only representative of the order living at the present day, although 

 over 1,000 different species of Nautilidae and Orthoceratidse were found 

 by Barrande in the Silurian basin of Bohemia alone. The Ammonites, 

 which are closely allied to the Nautilidae, were also very varied in type. 

 The condition of the eyes in the extinct Nautilidae and Ammonites is 

 not certainly known ; it is probable, however, that they were of the same 

 simple type (see Chap. 3, p. 48, and Figs. 112, 113) as in many of the 

 simpler univalve molluscs and their living representative Nautilus. 



1'ilLL. 



Fig. 112. — Section of Eye of Nautilus, showing i : Cavity of Optic Pit; 

 2 : Layer of Rods ; 3 : Pigment Layer ; 4 : Layer of Sensory Cells ; 

 5 : Layer of Ganglion Cells ; 6 : Branches of the Optic Nerve ; 

 7 : Epidermis ; 8 : Nerves of Cuticle ; 9 : Opening by which the 

 Optic Cavity communicates with the Surrounding Sea- water. (After 

 Hensen.) 



c. ap. : central aperture. 



cc. : central- cavity. 



ep. : epithelium. 



f.o.n. : fibres of optic nerve. 



g. c. : ganglion cells. 



nf.c. : nerve-fibres of cuticular epithelium. 



p. : pigment. 



re. : sensory retinal cells. 



rh. : rods. 



Moreover, the habits and the conditions of life of these animals, who lay 

 confined within a rigid shell in which they were carried about by currents 

 in the water, irrespective of any active or purposeful movements on their 

 own part, contrasts markedly with that of the rapid and active movements 

 of the carnivorous octopuses and squids in which highly differentiated 

 eyes have been evolved, and it seems probable that the restriction of move- 

 ment, combined with imperfect vision and almost complete dependence 

 for food on external circumstances, may have had a very considerable 



