146 S. WATARE. 



the effect of external stimulus. A further step might be a 

 depression in the skin at this point, which would serve some- 

 what to protect these diflferentiated and more sensitive cells, 

 while the deeper this depression the greater would be the 

 protection." 



That such steps of gradual development of visual organs 

 have actually taken place in some forms is quite probable. In 

 Arthropods, it seems to me worthy of remark that the omma- 

 tidium of the lateral eye of Limulus makes the nearest 

 approach to this primitive condition. It is nothing more and 

 nothing less than a depression in the skin, with the thickened 

 chitinous cuticle fitting in the open cavity, and acting as a lens 

 to condense the light. The cells which form the sensory part 

 of the structure are modified ectodermic cells, and, like the 

 rest of the ectodermic cells lying on the surface of the body, 

 secrete the chitinous cuticle on a part of their surface. 

 Assuming the surface where the chitin is secreted to be the 

 exterior, we may describe the ommatidium of the lateral eye 

 of Limulus as a group of modified ectodermic cells aggregated 

 around and beneath the funnel-shaped depression in the 

 skin. A glance at the diagrams (PI. XIX, figs. 1 and 2) 

 will show this clearly. The sensory cells of the ommatidium 

 come into direct contact with the conical lens, which is the 

 thickened part of the general cuticle ; or, to express this in 

 the phraseology of Lankester and Bourne, the ommatidium in 

 the lateral eye of Limulus is '' epistatic." The cornea 

 and crystalline cone as such have no separate existence in 

 this stage. 



Suppose such an ommatidium to become duplicated until a 

 considerable number be formed, as we may safely imagine to 

 have been the case, from the general tendency in the perfection 

 of a visual organ. What will be the result ? The first eff'ect 

 of such an increase in the number of ommatidia in a given 

 area will be the lengthening of each unit in the direction of 

 the ommatidial axis, and the cells (PI. XIX, fig. 3, V.) 

 which were situated directly on the outside of the retinulse will 

 travel over and above the sensory portion {Rt. and G., fig. 3). 



