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 [283] and 
reproduced in Pl. 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 Grenacher’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 
Descriprion OF Plate XXXVIII. 
The details of the compound eye : 
Fic. 1.—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 (72). 
Fic, 2.—The dioptron of a Blow-fly seen from its inner surface, showing the sclerite 
in the margin of the basilar membrane ; 7 ¢, ciliary muscle; ¢, tracheze sur- 
rounding the retina. The retina and all the nervous structures have been 
removed. 
F 1G. 3.—Two of the ommatea of a Plume Moth (Pterophorus) showing the lens-like 
rhabdomes. cc, correz ¢ , crystalline cone; /, corneal lens; # 6, membrana 
basilaris. 
Fic. 4.—A section of the eye of Copilia (after Claus copied from Exner [252]). ¢ 
cornea ; /, lens; wv, vitreous space; 7/, rhabdome ; 77, retina; 7, optic nerve. 
The nomenclature of the structures is in accordance with my own views. 
Fic. 5.—Modifications of the nerve end organs of the retina. a, dissociated 
twin cones from the retina of a Blow-fly ; 4, a group of similar cones from the 
retina of a Cockroach (Periplaneta orientalis); c, vacuolated rods from the 
retina of a Blow-fly, osmic acid preparation; d, isolated rods from the retina of 
a Noctuid Moth (probably Ay/ophasia sp.) ; ¢, two of the retinule of a Sphinx 
Moth (Acherontia sp.), 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. 
Fic. 7.—Portions of tangental sections through the dioptron of a Blow-fly just 
before the end of the pupa stage. 4, the pseudo-cone ; 4, iris cells; C, section 
through a group of rhabdomes ; , pigment fringes ; 7/, rhabdome ; ¢, tracheal 
vessels. 
Fic. 8.—A radial section of the eye of a Noctuid Caterpillar copied from my paper 
in the Linn. Soc. Trans. [233]; c, cornea; /, lens; 72, rhabdome ; 77, retina ; 
fp, pigment cells, and 7, optic nerve. 
Fic. 9.—A diagram showing the manner in which the images are formed, according 
to the dioptric theory, by an ommateum.  » 2, sub-corneal image, and 7 2, 
retinal image. 
