ve ; á K ie ‘ eri’ Li ; 
=~ was a sincere conviction in the truth of such principles that led 
592 The Sensory Organs. [September, 
Boll also, although he determined the direct nervous continuity of 
the hair-cells in the ears of these animals, did not establish the 
nervous connection of the hair-cells in the epithelium. Flem- 
ming described long central outrunners, which can be drawn out 
from the sub-epithelial tissue to a comparatively great length, and 
which become more or less varicose in preparations treated with 
potassium bichromate and osmic acid. He also describes the 
. sensory cells of pulmonate snails, but in these the hairs do not 
reach above the surface of the epithelium, apparently a modifica- 
tion to suit external conditions. 
Judging from the above, we have, to say the least, good grounds 
for including the percipient structure of the organ of sight in the 
same generalization as that of the other sensory organs. As re- 
gards the other layers of the retina, may they not be simply of 
ganglionic value? The comparative simplicity of the retina of 
molluscs seems, in cephalopods, to be counterbalanced by the ex- 
istence of a ganglion just before the optic nerve enters the eye. 
Again, the ganglionic character of the olfactory lobes may be of 
equal value. Indeed, this comparison might assume importance 
when we remember that both the olfactory lobe and the retina 
are processes of the cranial vesicles. 
4 
Conclusion —In casting a last glance at the sensory organs they 
seem to arrange themselves more or less readily into groups. 
Thus, the side organs or mucous canals, the Savian vesicles and 
the organ of hearing, seem to constitute a sub-division by them- — 
selves. The organs of taste and smell seem to constitute another, 
while the eye seems to occupy a third sub-division alone. With 
regard to other sensory structures it would be hazardous to make : 
any suggestions. ; 
-On the ground of certain principles that I have expressed, and- 
of others even far more general, we may be led to suspect from the i 
very beginning the common genesis of sensory organs ; and it 
me to read the literature bearing immediately upon the subject, 
and to learn in how far such an opinion is supported by fact. 
= How much it is supported by the results of the investigators 
which I have adduced, and the possible relations I have pointed 
_ out, I will leave you to judge. ; oe 
{Nore. : The literatur: referred to in thi paper is that only whic 
ains directly to the questions in point. One or two pap 
