AND EMBRYOLOGY OF LIMULUS. 23 



Farther evidence that the Trilobite's eye was constructed on the same ^^attern as that of 

 the living horse-shoe crab is seen in the sections made by Mr. Walcott. We will first 

 describe, briefly, the eye of Limulus. Plate 6, fig. 1 represents a section through the cornea 

 of Limulus ; cor, the cornea which is seen to be a thinned portion of the integument ; nc, 

 indicates one of the nutrient or pore canals, which are filled with connective tissue extend- 

 ing into the integument from the body cavity ; cl, is one of the series of solid conical corneal 

 lenses. These are buried partly in the black retina, and the long slender optic nerve just 

 before reaching the eye subdivides, sending a branch to each facet or cornea, impinging 

 on the lens. Fig. 6 represents a vertical view of the corneal lenses or facets, magnified 

 fifty diameters, as seen through the transparent cornea. It will be seen that they are 

 slightly hexagonal and arranged in quincunx order ; their external surface is flat, though 

 that of the ocelli is slightly convex. 



Now if we compare with the horse-shoe crab's eye that of the trilobite, Asaphus gigas, 

 (plate 6, figs. 8, 9), we see that the eye is raised upon a tubercle-like elevation of the cara- 

 pace ; the integument (m<) is about as thick as that of Limulus, and it contains similar pore- 

 canals {pc) ; the eye itself or cornea, occupies a rather small area ; its exterior surface, 

 instead of being smooth as in Limulus, is tuberculated, or divided up into minute convex 

 areas; these convexities are the external surfaces of the corneal lenses, which extend 

 through the cornea, so that its surface is rough instead of smooth as in Limulus ; cl indi- 

 cates one of the corneal lenses which are arranged side by side ; they are of slightly dif- 

 ferent lengths and thicknesses, and the rather blunt free ends project into the cavity of 

 the eye, which in the fossil is filled with a translucent calcite. 



It is quite apparent that we have here the closest possible homology between the hard 

 parts of the eye of Limulus and of Asaphus. Another point of very considerable 

 interest is a tolerably distinct dark line (fig. 9, rt), which seems to run across from one 

 lens to another, and which may possibly represent the external limits of the retina or 

 pigment mass in which the ends of the lenses were probably immersed ; should this be 

 found to be the indication of the outer edge of the retina, it would be a most interestmg 

 fact in favor of our view of the identity between the eyes of the two types of Palaeo- 

 carida under consideration. 



Another section sent us by Mr. Walcott is represented by plate 6, fig. 8 ; it is from 

 Ascqjhus gigas, but represents a less elevated and broader part of the eye than that seen 

 in j^late 6, fig. 9 ; the section does not so well exhibit the free ends of the corneal lenses. 

 Fig. 7 a represents a tranverse view of the eye of Ascqjhus gigas, showing the hexagonal 

 form of the facets, and their quincunx arrangement; so much like that of Lunulus 

 (fig. 6). 



This hexagonal appearance of the corneal lenses is still retained in natural vertical sec- 

 tions of eyes of the same genus, where with a good ToUes lens the sides of the cones are 

 seen to be angular. Plate 6, fig. 10, represents a few such cones. I do not understand to 

 what this hexagonal appearance is due ; for both in Limulus and the Trilobites the corneal 

 lenses appear usually to be round, and yet in making a" camera drawing (as are 

 all those here represented) of the cornea of Limulus from above, they present the same 

 hexagonal appearance as in Trilobites. The cause of this I leave to others to explain. 



