The Internal Anatomy of Spiders 



the eyes of this type are termed prcbacillar (pre-bac'il-lar) eyes. 

 In the other type the nuclei lie behind the optic rods; such eyes 

 are termed postbacillar eyes. 



The anterior median eyes are postbacillar; all other eyes of 

 spiders are prebacillar. 



In the prebacillar eyes (all eyes except the anterior median) 

 the nerve fibres join the visual cells at the end of the cell bearing 

 the rod (Fig. 169); while in the postbacillar eyes (anterior median) 

 the nerve fibres join the visual cells between the nucleus and the 

 rod (Fig. 167). 



The two types of eyes differ in the method of their develop- 

 ment, but this is a phase of the subject into which we cannot 

 enter here; it is sufficient to state that on account of their peculiar 



Fig. 160. DIAGRAMS OF A PREBACILLAR EYE 



c, cornea chy, corneal hypodcrmis r, retina e c, eye-capsule o n, optic nerve 



«', iris pigment (after Widman) 



method of development the anterior median eyes are termed by 

 some writers the inverted eyes, while all the other eyes are termed 

 erect eyes. 



The eye-capsule. — The eye-capsule is a sheath enclosing the 

 eye. It appears to be continuous with the sheath of the optic 

 nerve; it is sometimes termed the post-retinal membrane. 



The tapetum. — In the eyes of many animals there is a struc- 

 ture that reflects back the light that has entered the eye causing 

 the well-known shining of the eyes in the dark. This is often 

 observed in the eyes of cats and in the eyes of moths that are 

 attracted to our lights at night. The part of the eye that causes 



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