BY R. VON LENDENFELD, PH.D. 731 



of an outer layer of low cylinder cells, between which there are 

 slender sensitive cells, particularly abundant round the eyes or on 

 the sides of the tubercles (blind species.) 



5. The otolith-like concretions in tlie numerous vesicles of the 

 dorsal skin are composed of carbonate of lime and homologous to 

 parts of the shell of other related pulmonates. 



6. The eyes multiply by division, semi-detached eyes, and such 

 with a simple spherical pigment layer, but with two lenses are not 

 rare. 



7. The lens consists apparently of one single cell which retains 

 its nucleus and vitality, and may divide into two. A sphincter- 

 shaped circular accommodation muscle is clearly visible. 



8. The retina is of a much more complicated nature than 

 Semper, who only had spirit specimens at his disposal, was able to 

 discover. The radiating fibres of the nervus opticus are inter- 

 spersed with small ganglia cells. Below these follow cells with 

 peculiar plano-concave bodies in them, which are highly refractive. 

 These cells are broad and cylindrical. The final branches of the 

 Opticus extend downward between them to a layer of multipolar 

 cells below. Below this layer of granular (osmic acid) ganglia cells, 

 cylindrical and very regular hexagonal cells are found ; the axis 

 of each is situated in the direction of the entering light. These 

 hexagonal cells are attached to the pigment-skin at the outer limit 

 of the retina. Pigment granules extend up the sides of the 

 partition walls of these hexagonal cells for some distance. The 

 walls themselves are thickened below, and in this way concave 

 spaces are formed, one at the bottom of each hexagonal cell. 

 These spaces are completely surrounded by pigment. 



In the centi'e of each, a rod (Stabchen) is situated. This has a 

 conical shape, is attached with a broad base to the bottom of the 

 concave space, and tapers rapidly at first and more gradually after- 

 wards, to a fine point. Its faces Appear concave. The upper 

 pointed end of the rod is continued in the shape of a very fine 

 thread extending through the centre of the hexagonal cell and 

 joining the ganglia cell-layer. 

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