Eyes of Mollusca and Arthropods. 703 



Cardium , Pecten , and nocturual , or deep sea Vertebrates) . Therefore 

 under tliese conditious. the accessoiy eyes would also develop to a 

 high degree such refleeting surfaces. These reflecting- bodies are usu- 

 ally combined with highly refractive coudeusing lenses , and both 

 together give the orgaus thus i)rovided a very brilliant sparkle or 

 glow, whieh may be mistaken for iudependent lumiuosity. The 

 lateral eyes of Euphausia are provided with a remarkably well devel- 

 oped argentea and lens. 



It is extremely probable that the retina of the Vertebrates 

 consists of highly modified ommatidia, whose retinulae have lost 

 their pigment and become transformed into ganglionic cells. Some of 

 these ganglia , — Landolt'sche Keule and their homologues, — retaiu 

 their primitive position around the base of the retinophorae , or rod 

 cells, and constitute, with the nuclei of the latter, the nucleated layer 

 between the rods and the basal plexus. The fibres of the latter layer 

 may, like the basal brauch of the optic nerve in Pecten, consist only of 

 axial fibres , and like them enter the retinophorae and extend through 

 the centre of the rods as axial nerves. The ganglionic layer of the 

 retina is produced by the modification of retinulae into ganglionic cells 

 which , iustead of wandering away from the retina to form an isolated 

 optic ganglion, remaiu in close proximity to the sensitive rod cells. An 

 axial nerre of the rods has already been described by several authors, 

 and , as the Statements have been received with more or less doubt , it 

 may be worth while to say that I have been able to coufirm the state- 

 ment by Observation made upon the macerated retina of the dog -fish. 

 The basket-work of fibres on the surface of the rods is too well known 

 to require any confirmation. Max Schultze (Stricker s Handbuch der 

 Histologie) considers these fibres, which have been isolated toward the 

 base of the rods. as forming a part of the connective tissue skeleton of 

 the retina. Judging entirely from the facts obtained by a study of the 

 rods in the Invertebrates, we consider the external fibres in the Verte- 

 brate rods to be exactly similar to those found on the surface of the 

 rods in Pecten, i. e. ganglionic nerve fibres. The nerve fibres in 

 the centre of the rods in both groups, Vertebrates and Mollusca, 

 would be axial nerves. To carry out the comparison to its full extent, 

 the lamellae of the Vertebrate rods must be produced by the successive 

 e tag es of fibrillae which radiate from the axial nerve toward the peri- 

 phery of the rod, there unitiug with fibrillae from the external fibres. In 

 other words, a rod of the Vertebrate retina contains a retini dium ex- 

 actly similar to that found in the rods of the Mollusca and Arthropods. 



