COORDINATION 



215 



a limited area of ectoderm, the cells of which become differentiated 

 for receiving the photic stimuli that produce nerve impulses to 

 be transmitted to the central nervous system. Among Vertebrates 

 the sensory cells are also of ectodermic origin, but only secondarily 

 so, since the optic vesicles arise as outpocketings directly from 

 the sides of the diencephalon. (Fig. 149.) 



A retina alone, such as exists in some of the lower Invertebrates, 

 can afford no visual sensations other than light and darkness, and 

 perhaps in some cases the ability to distinguish light of one color 



Posterior chamber, 

 Anterior chamber 



(both filled with 

 aqueous humor) 



Eyelid 

 Eyelash 



Pupil 



Conjunctiva 

 Cornea- 



Iris 



Lens muscles 



Suspensory 

 ligament of lens 



Fig. 150. 



Muscle to 

 upper lid 



Optic 

 nerve 



Retina 

 Choroid coat 

 Sclerotic coat 



Muscle to eyeball 



The Vertebrate eye (human) . Vertical section of the eye and 

 associated structures. 



from that of another. In order that not merely degrees of the 

 intensity of light may be perceived, but that objects may be seen, 

 many of the higher Invertebrates have developed various kinds 

 of complicated apparatus for bringing the rays from a given point 

 to a focus at one point on the retina. These culminate, on the one 

 hand, in the compound eye of the Arthropods; and on the other, 

 in the 'camera' eye of certain Molluscs such as the Squid. In the 

 latter case the mechanism is very much like that found in the 

 Vertebrates, but since it occurs in Molluscs which cannot be con- 

 sidered in the direct evolutionary line of the Vertebrates, it 



