the Sense of Sight. 165 



afforded. The eyes in certain Platodes, and apparently also 

 in Leeches, are essentially of this type. 



3. In cases where a rigid chitinous cuticle is present, the 

 different ways in which the cells containing the granules 

 become associated with the sensory nerves are very numerous. 

 These cells may belong either to the skin or to the connective 

 tissue, or are, perhaps, collections of the wandering-cells 

 themselves. Leaving these differences on one side, we may 

 roughly divide the eyes arising under chitinous cuticles into 

 two groups — those in which the sensory nerves running close 

 under the cuticle turn round and face the advancing crowds 

 of granule-bearing cells or granules, and the other in which 

 the sensory nerves run with the stream towards the light. In 

 both these divisions w^e have many specializations of the 

 different cells composing the ultimate complex or organ, while 

 in all cases the excess of the advancing granules, which nor- 

 mally contribute an important constituent to the hard cuticle, 

 leads to the formation of rods, rhabdomeres, crystalline cones, 

 and lenses. We have, then, here to assume that individual 

 granules, though evidently impeded by the structure of the 

 complex, continually succeed in their efforts to advance and 

 in some way end by fulfilling their normal destiny of helping 

 to build up chitin. 



The secondary character of visual organs seems to follow 

 from the fact that eyes showing both the positions of the 

 sensory (retinal) cells described under this heading occur in 

 one aTid the same animal class, e. g. in the Arachnida and in 

 the Mollusca. 



Before passing on to the typical complicated vertebrate eye 

 I may refer to the persistent remains of a far simpler kind of 

 eye which are still to be found among lower vertebrates. I 

 refer to the ancient pineal eye, which is still recognizable as an 

 eye in the Lizards. This pineal eye, if its present condition, 

 say in Hatteria^ gives any evidence of its original morphology, 

 and it would be difficult to prove the contrary, is of the iirst 

 and simplest of the types above described *, and thus mor- 

 phologically, as well as phylogenetically, is a very primitive 

 structure. The pigment-granules or granule-bearing cells 

 which, in the functional eyes of that type, contribute to the 

 formation of the cuticular or gelatinous dioptric apparatus, in 



* I have to thank my frieud, Mr. Martin Woodward, of the Royal 

 College of Science, for the loan of sections of the pineal eye of Hatteria, 

 which leave no doubt in my mind on this point. I may add that I am 

 gi'eatly indebted to other hue preparations made and liindly lent me by 

 Mr. \\ oodward ; they have confirmed in a striking- manner many points 

 in my tlieory. 



