A ''L'nincj Fossir 233 



For, by covering the eyes of a pair which are obviously 

 about to leave the water, and turning them in the oppo- 

 site direction, we find that they soon arrive at their 

 destination regardless. . 



That Limulus can detect light from darkness, there / 

 is, of course, no question. This is borne out by the very 

 structure of both the compound and the simple eyes. 

 But this is not seeing, in the sense that humans or the 

 higher animals see. Externally, the compound eyes ap- 

 pear to be divided into a number of circular facets, each 

 barely touching its neighbor. Every facet is the cover- 

 ing of a unit that is a complete eye in itself. In sub- 

 stance, the individual unit is composed of the cornea, 

 which is simply the crust specialized by becoming trans- 

 parent, and the cone, or crystalline lens. It is this lens 

 that in the eyes of most other animals forms the optical 

 image; but in Limulus it seems to be a very imperfect 

 affair. A continuation of the crust invests the cone 

 which is perforated at the apex, and this in turn is ter- 

 minated by a rod surrounded with nerve fibrils arising 

 from the optic nerve. The inside of the cone is lined 

 with an opaque pigment that prevents the rays of light 

 from passing from one facet eye to another. The 

 ocelli, or simple eyes, are still simpler in their construc- 

 tion. They are in truth quite primitive. Their pig-^^ 

 mentation is irregular and they lack the nervous 

 organization of the compound unit. One peculiarity 

 of compound eyes in general should be mentioned here. 

 It is well known that many insects and crustaceans have 

 remarkable vision. Now, in all those cases in which 

 good vision Inheres, the facets of the eyes are arranged 

 in a geometrical pattern as if from compression; viz., 



