Eyes of Molluscs and Arthropods. G57 



probable, from my owii studies and from what I can gather from bis, 

 tliat büth pigmentcd and colorless cells form independent rods, and 

 that what he has describcd as a single one is really the produet of an 

 entire omniatidium ; the core and sheath of bis rod being due to the 

 arrangement of the cells composing the ommatidium. The rod of the 

 retino})hora, or colorless cell, forms the core of Hilger.s rod, while 

 those of the surrounding pigment cells form the sheath. 



Whether there are Gasteropod eyes in which only the retino- 

 phorae bear retinidia, while the pigment cells are modified for 

 other purposes, is not known at present. In the Heteropods, however, 

 according to Hensen, it looks as though the r et inula e were reducedto 

 very thiu pigmented cells at the outer ends of the retinophorae ; the 

 latter alone appear to develop rods. 



In the Cephalopods, as we bave already pointed out, the retino- 

 phorae (retinal cells) alone produce double rods, which may be com- 

 pared directly with the double rods of Area, Pecten, etc. The retinulae 

 bave become transformed into cells homologous with the ganglionic ones 

 of Pecten. In the latter case , the Inversion of the rods necessitates 

 the absence of pigment in these cells, while in the Cephalopods, wbere 

 the original direction of the rods is retained, no suchchange isnecessary. 



The pseudo-lentie eyes of ^rca must be regarded as small 

 aggregations of ommatidia on a nearly level surface, the retinidial 

 cuticula of which has become thickened into a lens-shaped body richly 

 supplied with nerve fibrillae, — in fact exactly what would be produced 

 by pulliug an invaginate eye out fiat. One might consider them as in- 

 cipient invaginate eyes, the retinal cuticula of which has increased in 

 thickness more rapidly than the ommateal layer has become invaginate. 



The retinated type of eyes is confined, as far as we kuow at 

 present, to Pecten, whose retina is characterised by the great develop- 

 ment of the retinophorae and their rods, and the complete absence 

 of pigment from the retinulae. The sensitive layer of the Cephalopod 

 eye must be regarded as a retineum in which the retinulae bave ceased 

 to produce rods. some being modified to form the colorless ganglionic 

 cells (»Sockel« cells of Gkenacher). The eyes of Cephalopods are 

 remarkable on account of their complicated dioptric apparatus, rather 

 than for any great specialization of the sensitive layer. 



Before making any comparisons between the retina of Pecten and 

 that of Cephalopods, I will attempt to explain my idea of the origin of 



Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. VI. 44 



