LATERAL AND CENTEAL EYES OF SCOEPIO AND LIMULUS. 187 



Von Graber, that a pavement epithelium is disposed on the 

 deep face of the cuticular lens. 



Von Graber^s statements, — Of the numerous points con- 

 cerning which Von Graber has made erroneous statements in 

 his writings on Scorpion's eyes,^ the most important is that 

 which relates to the fundamental structure of the lateral eye. 

 Von Graber states (and emphasises his statement by a drawing 

 professing to be an accurate copy of a preparation of the lateral 

 eye of Scorpio Europseus, Schr.) that the lateral eyes of the 

 Scorpions are provided with two rows of cells — a vitreous body 

 and a retinal body — ^just as are the central eyes, the two rows 

 of cells being separated by a membrane. This statement is alto- 

 gether erroneous. The description and figures which we here 

 publish show that the onimateum of the Scorpion's lateral eye 

 has no " vitreous body," and consists of a single layer of cells, 

 some larger (nerve-end cells), some smaller (interneural and 

 perineural cells). 



Von Graber's error in this matter has apparently arisen, like 

 most of the errors to which he commits himself in the same 

 memoir, from the defective character of his methods of investi- 

 gation. His sections were too thick and ill directed, and his 

 macerating and decolorising fluids were allowed to act too 

 rapidly or for too long a period. 



A further error of Von Graber in regard to the lateral eye is 

 his description of the ommaieum as composed of three layers 

 (besides his non-existent vitreous body) — a layer of nerve- 

 fibres, a layer of ganglion cells, and a layer of " rod cells " 

 (nerve-end cells). There are no " ganglion cells" within the eye- 

 capsule distinct from the nerve-end cells. Von Graber holds an 

 altogether erroneous view as to the structure of the nerve-end 

 cells, which, in opposition to Grenacher (who has studied and 

 described these structures in other Arthropoda), he declares to 

 possess three nuclei — an anterior, a middle (that of the rod cell 

 or rod region), and a posterior (that of the ganglion cell). The 

 nerve-end cell is thus, according to Von Graber, a compound 

 body, consisting of three fused cells. He terms it a " retinal 

 ^ ' Archiv f. Mikrosk. Anal.' vol. xvii, ISSO, p. 58. 



