1878. | The Sensory Organs. 591 
In all the retinze of vertebrates two elements among others ap- 
pear to be constant, namely, the layer of rods and cones and one 
corresponding to the layer of external granules. Each rod and 
each cone is thus connected with a cell element, and certainly it 
requires no great stretch of the imagination to see here a struc-. 
ture corresponding to a hair-cell. We have a cell with a central 
and peripheral process which last bears an apparatus to receive 
the immediate impression of force. Besides evidence by no 
means ends here. In those invertebrates whose eyes approach 
and even exceed in their complexity those of vertebrates, as the 
gastropods and cephalopods, the same elements remain constant, 
namely, the rods with their cells (Schultze, A. f. M. A. v, 1869), 
(Babuchin, Würzburg Verh’dlg, 1865) (Hensen Z. f. w. LH, 
1865). 
We must be: also that in the eyes of molluscs the re- 
lation of the rods to the eye is exactly the reverse of that in ver- 
tebrates ; namely, they point directly forwards and constitute the 
innermost layer of the retina, while in vertebrates they point 
directly backwards and constitute the outermost layer. This fact 
assumes great significance when we learn what is known concern- 
ing the development of the eyes of molluscs. Hensen says (Z. 
f. W. Z. xv., 1865), that the eyes of cephalopods probably origi- 
nate in the same manner as the ear and nose of vertebrates, 7. e., 
by an inflexion of the epithelial layer. In the nautilus, he says, 
it is evidently the case, while for the gastropods this has. actually 
been observed in a pulmonate snail of the Phillipines; by C. Sem- 
per’ (quoted by Hensen). 
When we are told by Boll (A. f. M. A. v, 1869, supplement), 
that the epithelium of molluscs everywhere contains hair- 
cells continuous with nerves, it would seem to place the morpho- 
logical value of the rods and cells in these animals beyond a doubt. 
Boll describes what he calls the “ neuro-epithelium.” He pictures 
stiff, non-vibratile hairs, distinct from vibrating cilia, and sur- 
mounting elongated nucleated cells. They occur all over the sur- — 
face of gastropods and cephalopods, being more thickly situated 
in parts used for apprehension or contact, as the arms of the cuttle- 
- fish and the edges of the foot of the snail. Flemming (Ibid. v, 1869 
and vi, 1870) corroborates Boll’s statements in the main points, 
_ though he differs from him in the detailed description of the hairs, - 
a2 Semper has written a separate paper on molluscan eyes, which I could not obtain. -o 
