336 



INVERTEBRATE MORPROLOGtT. 



ment unusual for Invertebrates. The extremity of the pro- 

 cess is occupied by a number of clear transparent cells which. 

 serve as a cornea {c6) and which are continuous with a zone of 

 pigmented cells {pg) analogous to an iris, and which pass grad- 

 ually over into ordinary ectodermal cells. Upon the inner 

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

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

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



1 



Pig. 150. — Eye of P«c<«w (modiflefl 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 (p'p'), passing to one side of the eye, bends inwards 

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

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

 verted, the retina-cells being turned awaj^ 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 luciduni (tl), which serves as a reflector 

 and gives the metallic lustre which is characteristic of the 



