The Internal Anatomy of Spiders 

 b. THE VISCERAL NERVOUS SYSTEM 



A visceral nervous system has been proven to exist in spiders 

 and in other Arachnida. It consists of an unpaired nerve con- 

 nected with the brain by paired nerves and running along the 

 oesophagus and stomach. Ganglia connected with the ventral 

 chord have also been described as belonging to the visceral nervous 

 system (Lang). 



C. THE EYES 



The number and the position of the eyes have been discussed 

 in the opening pages of the preceding chapter; here we have to do 

 with their internal structure.* 



The eyes of spiders are o 

 the type known as the several 

 layered ocellus, each eye con- 

 sisting of two or more layers 

 of cells behind a single lens 

 (Fig. 167). The lens (Fig. 

 167, c) is termed the cornea; 

 the outer of the two more 

 prominent layers (Fig. 167, 

 c. by), the corneal bypodermis; 

 and the inner of these two 

 layers (Fig. 167, r), the retina. 

 The eye is enclosed in a 

 sheath (Fig. 167, ec), the eye- 

 capsule; in some eyes there 

 is a layer of reflecting cells 



c.hy 



o.n. 



e.c. 



Fig. 167. 

 DIAGRAM OF A POSTBACILLAR EYE 

 c, cornea chy, corneal hypodermis r, retina 

 ec, eye-capsule on, optic nerve (after W'idman) 



behind the retina, the tapetum; and in some eyes there are 

 pigment cells, which vary in position in different eyes. 



The cornea. — The cornea or lens of the eye (Fig. 167, c), is 

 merely a modified portion of the cuticula of the body-wall, with 

 which it is continuous. It is a lens-shaped thickening of the 

 cuticula, which is free from hairs and, consequently, is not 

 traversed by trichopores, and in which the primary cuticula is 

 devoid of pigment, permitting the passage of light to the nervous 

 portion of the eye. 



'Much has been published regarding the eyes of spiders. The works that I have consulted 

 most in the preparation ot the account given here are the following: Grenacher '70; Graber '8o; 

 Schimkewitsch '84; Bcrtkau '86; W'idman '07; and one. as yet. unpublished paper, l>v Mr (leorge 

 D. Shafer. 



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