'232 SUMMARY OF CURRENT RESEARCHES RELATING TO 



optic nerve, he says that they belong to the Vertebrate type of eye 

 (" typus der Wirbelthieraugen "), so that two animals are now known, 

 each belonging to a large and important class of Invertebrata (Gas- 

 teropoda and Lamellibranchiata respectively), that possess eyes which 

 are built up on this type. The eyes ofPeden are even more deserving 

 of the name of " Wirbelthieraugen " than those of Onchidium, for they 

 are much more highly developed, and possess, in addition to this 

 relationship of the nerve and retina, other Vertebrate peculiarities. 

 The lens is multicellular, a character which, although not unknown 

 amongst the Invertebrates, is much more characteristic of the Verte- 

 brata. The tapetum, too, a structiire which doubtfully exists in any 

 other Invertebrata, is found in Pecten and some Vertebrates. But, 

 although the application of this word " Wirbelthieraugen " to these 

 eyes is convenient for the adult condition, it must be carefully 

 remembered that the development of these eyes is essentially 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 invagination of this process. It is 

 impossible for the eyes of Pecten or Oncliidimn 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 morpho- 

 logical 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 high and dry on the shore, 

 they can appreciate 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. 



With regard to methods, the author says that for a general examin- 

 ation of the eye the best method is to harden in alcohol, and stain by 

 immersion in hfematoxylin for twenty-four hours. Of the osmic-acid 

 preparations the best were obtained by immersion in a 1 per cent, 

 solution for fifteen minutes, followed by absolute alcohol for three or 

 four days. This method is of great value for studying the retina and 

 lens. He also used gold chloride for staining the nerves with some 

 success. For examining the tapetum the best preparations were made 

 from some eyes given him 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 the eyes were allowed to remain in a 

 solution of chloral hydrate for four or five days. The retina was 



