SENSE ORGANS. 



337 



The optical arrangement of the lateral and median eyes of 

 Scorpions and Spiders and of the median eyes of Limulus is 

 such as to throw a reversed image on the retina. 



The development of the lateral eyes of the Scorpion supports 

 the view of Lankester and Bourne that they are derived from an 

 eye very like that of Limulus, the several elements of which 

 have become separate. It is easy to imagine, on purely theo- 

 retical grounds, that the compound eyes of Insects and the 

 Crustacea might be derived from an eye such as the lateral eye 

 of Limulus by the opposite process, namely by the specializa- 

 tion of the several depressions into ommatidia, their increase 



FIG. 237. A anterior, B posterior median eyes of a spider (diagrammatic from Korschelfc 

 and Heider, after Grenadier and Bertkau). ch chitinous cuticle, passing into cuticular 

 lens (I); gl vitreous body; h hypodermis ; I lens; n optic nerve; r retina; st rod; t 

 tapetum. 



in number, and closer aggregation. However the facts at our 

 disposal seem rather to point, as above indicated, to the origin 

 of compound eyes of Insects in a different manner, i.e. from 

 groups of distinct ocelli such as are found in Myriapods. The 

 morphological nature of the paired eyes of Crustacea is dis- 

 cussed on p. 249. 



Statocysts. The open or closed saccular organs of the Crus- 

 tacea, containing sensory hairs, have generally been named 

 otocysts. It is now recognized that they perform the function 

 of statocysts (cf. p. 350), though they may possibly possess 



z in z 



