482 Miscellaneous. 



On the Structure of the Ocelli of Lithobius. 

 By M. Victor Willem. 



The study of the ocelli of Lithobius forjlcatus has been the 

 object of researches by Graber and Grenacher ; but the descriptions 

 given by these two authors differ in all their details, so much so 

 that they seem, as Grenacher himself remarks, to have examined 

 different animals. 



Graber * states in effect that the visual organs of the !Myriapods 

 have an organization so similar to that of the eyes of Arachnids that 

 he deems it useless to give a special description of them. Now, 

 according to this author, the ocellum of an Arachnid comprises two 

 layers of cellular elements, separated by a delicate lamellar mem- 

 brane : — a complete layer of cells clothing the internal face of the 

 corneal lens and representing a vitreous body, and, in the second 

 place, a retina formed of elements directed parallel to the axis of the 

 eye. Each of these retinal elements must be considered as consti- 

 tuted by a basal ganglion-cell, the terminal prolongation of which, 

 or rod, is cajyped by a uni- or sometimes binuclear cell. 



According to Grenacher f there is found beneath the cornea a 

 circlet of large pigmented prismatic cells, forming around the axis 

 of the eye a hollow cylinder, the cavity of which is occupied by 

 transversely directed cilia converging from the internal margin of 

 the cells {Haarzellen) towards the axis of the visual organ. The 

 posterior part of the ocellum is occupied by a hcmisi)herical group 

 of unicellular pigmented retinal elements, each one of which is 

 terminated on the inner side by a rod, the structure of which is 

 extraordinarily difficult to elucidate. Lastly, behind the lens we 

 may observe a limited number of little cellular nuclei. 



A third observer, Sograff J, gives a vague and very summary 

 description of Lithobius, which does not accord with either of the 

 foregoing. 



In spite of the numerous difficulties which this study presents, I 

 have succeeded in obtaining satisfactory sections of this organ, and 

 have found that their structure corresponds, at any rate broadly, 

 ■with the description given by Grenacher. 



Each of the ocelli has the form of an elongated cylinder, bounded 

 externally by the cornea, and surrounded by a connective membrane 

 which is traversed by the optic nerve ; in the furro« s which sepa- 

 rate the corneal facets from one another this membrane is thickened 

 and encloses a number of little pigment-cells. 



* " Ueber das unicorneale Tracheaten- und spocioU das Arachnoidt-n- 

 und Mvriopodon-Aiige," Arcbiv fiirmikroskopit^che Anatomic, ITt^TBand, 

 1880. 



t " Ueber die Augen einiger Myriopodeu," ibid. 18*^' Band, 1880. 



X ' Anatomic do Lithobius forJivatu»^ Moscow, 1880, p. 20 (iu Russian). 



