548 THE SENSES AND SENSORY ORGANS. 



and in this I agree with the authors cited, but I deny its pos- 

 sible retinal character. How Lankester could have thought 

 the organ represented in his figures, and especially in Fig. 20, 

 [229], could be a retina, is incomprehensible to me. A com- 

 parison of this organ in Limulus with the same structures 

 represented by me in the simple eyes of Caterpillars [233] and 

 reproduced in PI. XXXVIII., Fig. 8, renders it probable that 

 the true retinal end organs have been hitherto overlooked in 

 Limulus, and a comparison of these figures with Grenadier's 

 representation of an ommateum from Hyperia Galba 222, 

 Fig. 104], which is almost identical with my figure of the 

 eyes of Caterpillars, appears to me conclusive that the true 

 retina of Limulus has been overlooked. What Grenacher 



DESCRII-TION OF PLATE XXXVIII. 



The details of the compound eye : 



FIG. I. A portion of the dioptron of a Blow-fly Nymph on the ninth or tenth day 

 of the pupa state, showing the relation of the tracheal vessels (/;-) to the cells of 

 the sub-dioptric space (in}. 



FIG. 2. The dioptron of a Blow-fly seen from its inner surface, showing the sclerite 

 in the margin of the basilar membrane ; m c, ciliary muscle ; tr, trachea; sur- 

 rounding the retina. The retina and all the nervous structures have been 

 removed. 



FIG. 3. Two of the ommaiea of a Plume Moth (Pterophorus) showing the lens-like 

 rhabdomes. c, ccrrta e , crystalline cone ; /, corneal lens ; ;// b, membrana 

 basilaris. 



FIG. 4. A section of the eye of Copilia (after Claus copied from Exner [252]). c, 

 cornea ; /, lens ; r, vitreous space ; r/i, rhabdome ; rl, retina ; >i, optic nerve. 

 The nomenclature of the structures is in accordance with my own views. 



FIG. 5. Modifications of the nerve end organs of the retina. a, dissociated 

 twin cones from the retina of a Blow-fly ; />, a group of similar cones from the 

 retina of a Cockroach (Periplaneta orientalis) ; c, vacuolatecl rods from the 

 retina of a Blow-fly, osmic acid preparation ; </, isolated rods from the retina of 

 a Noctuid Moth (probably Xylophasia ./. ) ; f, two of the retinulre of a Sphinx 

 M"ih {Acherontia s/>.), from a large foreign pupa received in spirit; /, a tan- 

 gental section through the outer part of the retina of a Blow-fly pupa in an 

 advanced stage of development showing the reticulum formed by the chaplet 

 cells of Viallanes. 



FIG. 7. Portions of tangental sections through the dioptron of a Blow-fly just 

 before the end of the pupa stage. A, the pseudo-cone ; />', iris cells ; C, section 

 through a group of rhabdomes ; /, pigment fringes ; rh, rhabdome ; //-, tracheal 

 vessels. 



FIG. 8. A radial section of the eye of a Noctuid Caterpillar copied from my paper 

 in the l.inn. Soc. Trans. [233] ; <', cornea; /, lens; ;//, rhabdome ; ft, retina; 

 / c, pigment cells, and , optic nerve. 



Fin. 9. A diagram showing the manner in which the images are formed, according 

 to the dioptric theory, by an ommateum. s t, sub-corneal image, and r i, 

 retinal image. 



