336 



INVERTEBRATE MORPHOLOGY, 



inent unusual for Invertebrates. The extremity of the pro- 

 cess is occupied by a number of clear transparent cells which 

 serve as a cornea (co) and which are continuous with a zone of 

 pigmeuted cells (pg) analogous to an iris, and which pass grad- 

 ually over into ordinary ectodernial cells. Upon the inner 

 surface of the cornea is a mass of transparent cells constitut- 

 ing a lens (I), and below this lies the sensory portion of the 

 eye. The optic nerve as it conies towards the eye branches ; 



1 



co 



pg 



rt 



FlG. 150. EYE OP Pecten (modified slightly from PATTEN). 

 co = cornea. op, op' = optic nerve. 



I = lens. pg = pigment-cells. 



la = blood-lacuna. rt = retina. 



tl = tapetum lucid um. 



one branch (op'}, passing to one side of the eye, bends inwards 

 towards the axis of the eye between the retina-cells (rt) and 

 the lens. The sensory portion of the eye consequently is in- 

 verted, the retina-cells being turned away from the light which 

 must pass through the fibres of the optic nerve to reach them. 

 Below the retina and separated from it by a space is a layer 

 of tissue, the tapetum lucidum (tl), which serves as a reflector 

 and gives the metallic lustre which is characteristic of the 



