352 INVERTEBRATE MORPHOLOGY. 



external water through a small circular opening in the front 

 flattened wall of the cup. The eye is a camera constructed on 

 the " piu'hole " type, the image being defined by the exclusion 

 of all the more divergent rays of light which pass in from the 

 object towards the eye. 



In the remaining forms the eyes (Fig. 156, B) are large 

 globes imbedded in an orbit formed by the lateral portions 

 of the cephalic cartilage and its processes. The retinal por- 

 tion of the eye closely resembles that of Nautilus, consisting 

 of an external layer of rods {r) bounded beneath by a pigment- 

 layer {pg) beneath which is a nerve-layer (n) enclosed within 

 a connective tissue-sheath in which cartilage (c) is developed. 

 The optic nerve dilates into a retinal ganglion before being 

 distributed to the retina, the rods of which, it will be noted, 

 are turned towards the source of light. The eye-cup differs, 

 however, from that of Nautilus in being completely closed, 

 and the cells which form the outer and inner layers of the 

 outer wall of the cup secrete chitinous material which acts 

 as a lens (Z), forming a powerful biconvex condenser. In ad- 

 dition to this the eye is further complicated by the develop- 

 ment of a series of folds from the skin in its neighborhood. 

 One such fold is developed from the front wall of the optic 

 sac, surrounding the region occupied by the lens and form- 

 ing an iris {ir), the circular opening in its centre correspond- 

 ing to the pupil of the Vertebrate eye. A second likewise 

 forms nearer the base of the optic sac and, growing forward, 

 may enclose a space bounded behind by the iris and lens, 

 resembling the anterior chamber of the Vertebrate eye, the 

 portion of the fold immediately in front of the lens becoming 

 transparent and forming a cornea {co). The anterior chamber 

 is not, however, closed in all forms, but remains in communi- 

 cation with the exterior by an aperture produced bj' a failure 

 of the edges of the fold to unite completely. Finally, in some 

 forms other folds, which from analogy have been termed eye- 

 lids, develop. 



The resemblance of such an eye to that found in the Vertebrates is ex- 

 ceedingly striking, but a detailed study of the structure and mode of origin 

 of the various parts demonstrates conclusively that the similarities are ana- 

 logical only and not homological. One of the most important of the diflfer- 



