• 454 
SYDNEY J. HICKSON. 
different from that of the Vertebrate eye. The Vertebrate 
eye is formed in the embryo from a hollow process given^off 
from the brain^ and the future eye-cup is formed by an in- 
vagination of this process. It is impossible for the eyes of 
Pecten or Onchidium to be formed by any process similar 
to this. Thus^ in the young state these eyes are essentially 
different from those of the Vertebrata^ and the resemblance 
in the adult is merely accidental, and by no means due to 
morphological identity. 
Little is known and little can be said concerning the 
function of the eyes of Pecten. The presence of such a 
well-formed tapetum makes it probable that they are 
capable of appreciating very diffused light, and the close 
approximation of the lens to retina makes it exceedingly 
improbable that any image is formed upon the latter. 
A few experiments have been made on the extent of their 
visual power, which make it very doubtful whether they are 
of much value to the animal in avoiding its enemies. The 
most reasonable theory of their function seems to be that, 
when on the ebbing of the tide, a probability arises that 
they will be left higli and dry on the shore, they can appre- 
ciate the fact by the growing intensity of the light, and, 
by that peculiar flapping motion of their valves the 
Pectens are so remarkable for, move away into deeper 
water. ' 
These researches were entirely carried on in the morpho- 
logical laboratory of the University of Cambridge, and my 
best thanks are due to Mr. Balfour for his valuable advice 
and encouragement during the whole course of my researches. 
Owing to his kindness, also, I have been enabled to ex- 
amine some of Semper’s preparations of the eye of 
Onchidium, to which reference has been made in the text. 
Methods , — For a general examination of the eye the best 
method is to harden in alcohol and stain by immersion in 
haematoxylin for twenty-four hours. Of the osmic-acid acid 
preparations the best were obtained by immersion in 
a 1 per cent, solution for fifteen minutes, followed by abso- 
lute alcohol for three or four days. This method is of great 
value for studying the retina and lens. I have also used 
gold chloride for staining the nerves with some success. 
For examining the tapetum the best preparations I have were 
made from some eyes given me by Mr. Haddon, which had 
been treated with picric acid. This reagent seems to have 
dissolved away the red pigment, and consequently left the 
tapetum free from the numerous little red granules which 
