20 A. S. PACKARD, JR., ON THE ANATOMY 



The subdivisions of the optic nerve can rarely be traced for a long distance contin- 

 uously, owing to their irregular, tortuous course. In the drawing (fig. 1), I have 

 delineated with the aid of the camera lucida an actual section ; the clear spaces indicate 

 the cut portions of the nerves distributed to each corneal lens. Histologically they 

 present the same appearance as the nerve-fibres in the brain, those given oif from the 

 lower ganglionic cells. Under a low power (| inch), they appear to be structureless ; 

 under a | they are seen to be finely granulated. After repeated search I could find no 

 nuclei in these fibres ; nor were there any ganglion cells to be discovered. Repeated exam- 

 inations of numerous sections treated in different ways, have convinced me that there 

 are throughout the subocular area no ganglion cells, such as are characteristic of the eyes 

 of spiders and myriopods ; hence, with Grenacher, we may state that a ganglion ojMcum 

 is entirely wanting in Limulus ; the irregular, tortuous subdivisions of the optic nerve are 

 sent directly to the corneal lenses. 



Coming now to the structure of the individual eye, or facet in the compound eye, we 

 find that its anatomy is just as described by Grenacher, except that we have been led to 

 doul^t the existence of the layer of triangular (in outline) cells, which he represents as 

 extending up between the conical corneal lenses, and it should be borne in mind that we 

 examined eyes taken directly from living Limuli, as well as specimens that had been 

 preserved in alcohol for several years. Grenacher's researches were made on eyes pre- 

 served for a long time in specimens of Limuli obtained from German museums, and his 

 material w^as so poor that he did not attempt to study the simple eyes (ocelli). 



The structure of the cone of pigment matter enclosing and extending beyond the 

 end of the conical line has been described and illustrated in a masterly manner by 

 Grenacher. Impinging on the end of the conical lens, and extending through the centre 

 of the conical mass is a twelve-radiate semi-solid body, called by Grenacher the rhabdom, 

 and w^hich he appai-eutly homologises with the rhabdom or spindle-shaped body, succeeding 

 the rod of the ordinary Crustacean eye. Along a part of its length, this rhabdom 

 (fig. 4, rhah) is enveloped by the retinula (plate 6, fig. 4, ret). Our figures show in 

 sections the rhabdom, with its central axis and twelve or thirteen rays, forming a rosette 

 extending into the substance of the retinula. That the retinula is, as Grenacher figures, 

 composed of as many large cells as there are rays of the rhabdom, we have proved by 

 preparations treated with acid, as seen in fig. 4, a. 



How the optic nerve is connected with or impinges on the rhabdom, we have been 

 unable to ascertain. We have only seen enough to convince us that the nerve reaches 

 the end of the rhabdom, but the nature of the ending is unknown to us. The nerves, 

 as seen in our drawing, fig. 1, sometimes appear as if they ran by the end of the retinal 

 cones, and extended up between the corneal lenses. On the other hand, we have seen 

 very plainly the mode of termination of the nerve in the ocellus. Grenacher, however, 

 states that " a number of the nerve-fibres are distributed to each single-eye [facet], 

 they diverge behind it, and I have repeatedly traced clearly the entrance of a fibre into 

 the axial part of a retinula cell." 



Grenacher concludes that perception in the typical Arthropod eye is performed accord- 

 ing to the mosaic theory of Miiller, and that this applies to the eye of Limulus, although 

 the eye of the latter is morphologically wholly different from the eyes of any other animal. 



