12 SYDNEY J. HICKSON. 



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 lights 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 high 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 

 hsematoxylin 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 lers. 1 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 

 generally cling to it. For examining the isolated rods of the 

 retina I have allowed the eyes to remain in a solution of 

 chloral hydrate for four or five days. I have then dissected 

 out the retina with needles as carefully as possible, and 

 poured a drop or two of hsematoxylin on to the slide. When 

 the retina had been standing in hsematoxylin in this manner 

 for some hours it was washed with water, teased out with 

 fine needles, and mounted in glycerine. 



