266 THE RETINA, BY MAX SCHULTZE. 



the disks placed one behind the other increases, whilst their thickness 

 varies, though only to a slight extent. It would therefore seem tha 

 the reflection and modifications of the luminous waves resulting from 

 the disks, and having relation to perception on the part of the outer 

 segments, must necessarily, from their greater length, be here mud 

 more complete. In correspondence with their entirely different modt 

 of development, the structure of the retina of Invertebrata differs es- 

 sentially from that of Vertebrata. This is particularly noticeable ii 

 the layer of the perceptive elements, or bacillar layer, In Mollusks 

 Articulate animals, and Vermes, the extremities of the optic nerve form 

 as in the Vertebrata, a layer of palisade-like structures. Apparently 

 however, these are more favourably placed in relation to the light 

 than amongst the Yertebrata ; they are directed forward towards the 

 lens, whilst the bacillar layer in the Vertebrata is in contact with the 

 choroid. This difference is explained by the mode of development of 

 the retina in the two cases, being formed in the Vertebrata by an 

 e version of the central vesicle (see below), whilst in the Invertebrata 

 it originates from an inversion of the skin.* 



Amongst the Mollusca the structure of the retina is most exactly 

 known in the Cephalopoda and Heteropoda.f The innermost layer of 

 the retina, which is separated from the external layers by brownish- 

 black pigment, is formed by rod-like palisades that are of considerable 

 length in the Cephalopoda, and. are visible even to the naked eye as a 

 reddish lamina. The rod-like layer is composed 



1. Of lamellated palisades resembling the outer segment of the 

 rods of Vertebrata, though much more variable in the form they 

 present on transverse section, which may be semilunar, annular, 

 quadrangular, or quite irregular. Adjoining palisades may so coalesce 

 with one another as to form a continuous mass, traversed by 

 vertical tubes. The laminae that form these palisades, according to 



* Semper, according to a communication by Heiisen, Archiv fiir Mikros- 

 kop. Anatomic, Band ii., p. 416. 



t See Babuchin, Wiirzburg. Nat. Zeit,scfirift } Band v., p. 125, 1864 ; 

 Heiisen, Ueber das auge einiger Cephalopoda, " On the eyes of some Ce- 

 phalopods," Zeitschrift fur urissenschaft. Zoologie, Band xv. ; and Bronn, 

 Klassen und Ordnungen der Thier. Molusken, Taf. cxv. Steinlin, Beitrdge 

 zur Anatomie der fietina, St. Gallen, 1865-66, p. 70. Max Schultze, 

 Archiv fiir M'ikroskop. Anatomie, Band v., p. 1, Ueber die . Netzliant 

 anderer Mollusken, " On the retina of various Mollusks," etc. Babuchin, 

 Sitzungsberichte der Acad. zu VI ien, Juni, 1865 ; and Hensen, Archiv fiir 

 Mikroskop. Anatomie, Band ii., p. 399, in which last paper the literature of 

 the subject is fully given. 



