124 



THE PINEAL ORGAN 



although they may be regarded as intermediate in type do not indicate 

 a close relationship. The eyes of the ordinary garden and house spiders 



Fig. 86. 



Part of a section through the composite lateral eyes of a king crab (Limulus), 

 showing c. cp. : the chitinous carapace ; cp. : the ingrowing papilla-like 

 thickenings of this which lie over the retinulas, or separate eyes ; ret. : 

 retinula ; hyp. ; hypodermis, the cells of which are modified opposite the 

 papillae to form the sensory cells of the retinulas ; n. : nerves of the retinulje. 

 (After Lankester and Bourne.) 



are also very remarkable. They vary in number from one to six pairs, 

 four pairs being present in the leaping spider, Salticus scenicus, shown in 

 Fig. 88, in which they are seen to be situated on the carapace, the central 

 or frontal pair being the largest. The frontal eyes of Epeira diaderma 

 are of the simple, upright type ; whereas the posterior or lateral eyes show 

 what is described as an " inversion " of the constituent parts of the 

 retinal cells. The posterior eyes vary more particularly with respect to 

 the position of the nuclei of the retinal cells, relative to the refractile rods 

 or bacilli ; instead of being proximal or posterior to the rods, they are in 

 front of these, as in the lateral eyes of vertebrates. This position of the 

 nuclei of the retinal cells was termed by Graber " pr£e-bacillary," in 

 contrast to their " post-bacillary " situation in the simple, upright eye. 

 It must be borne in mind, however, that the retina in the inverted lateral 

 eyes of vertebrates is compound, whereas the lateral eyes of spiders, 



