AND EMBRYOLOGY OF LIMULUS. 25 



chitinous crust of the animal. In a specimen (fig. 12), not treated with acid, the end 

 of the cone is seen to be buried in j^igment, and in one out of many sections, i. e., that 

 figured 14, the cut went directly through the ocellar nerve, which, as seen in the 

 figure, after leaving the branch distributed to the other ocellus, proceeds undivided 

 to a distance about equal to the diameter of the corneal lens, when it gives off 

 minute fibres which pass up and lose themselves in the pigment layer near the base 

 of the cone. The main nervous trunk is seen to impinge dii-ectly on the end of the 

 pigment mass surrounding the end of the lens, while branches pass up into the pigment 

 on each side of the lens ; so that the latter is immersed, so to speak, in a multitude of 

 nervous fibres. 



On treating the pigment with acid, and cutting a section on one side of the solid lens, as 

 at plate 6, fig. 5, the entire mass of connective tissue and pigment is seen to be permeated 

 with nerve-fibres, which end in slight, bulbous, partly hyaline expansions next to the 

 chitinous integument. 



Nothing like the rhabdom or retinula was to be observed, and I doubt much if they 

 exist, or any nucleated ganglionic cells. 



We have, then, in the simple eye or ocellus of Limulus a repetition of the general 

 structure of any one of the individual eyes of the compound organ of vision, without the 

 rhabdom and retinula. The simple eye, then, in the horse-shoe crab is apparently rather 

 more rudimentary than one of the elements of the compound eye ; and it is difficult to 

 conceive of a much simpler form of eye in an arthropodous animal ; hence it can not be said 

 of the ocellus of Limulus that it is not less primitive in structure than tlie compound eye ; 

 for we have here the eye reduced to a corneal lens, retina and optic nerve, the simplest 

 association of elements in any organ of vision. 



Comj^arison of the ocellus of Limulus with the eyes of Ilyriojiods. When we compare 

 the ocellus of Limulus with that of the Arachnida, and of larval insects, there is 

 very considerable difference. In the form of the corneal lens, however, the ocellus 

 of Limulus somewhat approaches that of the Myriopods, as lately worked out by 

 Graber.-' 



An examination of the agglomerated eye^ of Bothrojwlys hipunctatus Wood — a 

 genus allied to Lithobius, the species here named being common in Northern 

 California at the base of Mount Shasta — shows us that the myriopod eye, as a whole, 

 is entirely unlike that of Limulus. 



The brain, in the first place, is on the usual Arthropod type ; the hemispheres beino- 

 symmetrical, and the relative position of the larger ganglion-cells being like those 

 of hexapod insects. A large mass of ganglion-cells is situated at the origin of 

 each optic nerve. As regards the eye or group of eyes, the individual eyes are abont 

 eighteen in number and closely aggregated, though each simple eye or facet is 

 circular and its surface convex. The cornea in the specimen examined, while externally 

 convex, is concave on the inside, the cornea being no thicker in the middle than 



1 Ueber das unicoreale Tracheaten- und speciell das Archiv. fiir mikr. Anat. Bd. 17, heft. 1. Aii<r. 14, 1879. 

 Araohaoiden- uQd Myrlopoden-Auge. Von V. Graber. '^ Our sections were kiadly made for us by Mr. Mason. 



