Eyes of Mollnscs and Arthropods. 569 



from their structure, having- each a minute nerve, apupil, a pigmentiim . 

 a striated body, and a lens, and from tlieir Situation at the edge of the 

 mantle, where alone such organs could be useful, and also placed as in 

 Gasteropods with the teutacles, must be organs of vision«. To his con- 

 cise descriptiou^ where by the »pigmentum« is meant the red pigment 

 layer, and by the »striated body«, the retina, not very much was added 

 until Hensen (12) published his paper of which we shall speak later. 



Krohn (3) and Grube (4) published, almost simultaneously, an 

 account of visual organs in the Lamellibranchiata, in which the optic 

 nerve and argentea were described with tolerable accuracy. To the 

 former is due the credit of having first seen the septum , a name 

 which he himself introduced. 



^Ye are indebted to the vivid Imagination of Will for the greatest 

 number of accurate observations, as well as for many mistakes, con- 

 cerning the visual organs of Lamellibranchiata. In some cases, he has 

 very accurately described the external characteristics of the eyes of 

 Area, Pectimculus and Cardium ; in others , he has scattered , with 

 lavish band, high sounding names to organs, which, if they once 

 existed , now seem to have disappeared. To him, however; is due the 

 credit of having first accurately described the cellular structure of the 

 lens of Pecten. The observations of Will appear to bave been accepted 

 without comment by Siebold, Leydig, Bronn, and even by still later 

 writers . 



Keferstein (11) recoguized in the so-called viteous body, the fib- 

 rous structure of which had been known since Garner, the real retina, 

 the rods of which he believed to be turned inwards, as in Vertebrates. 



The classical researches of Hensen carried our knowledge, at one 

 bound, to the position which it occupies to-day. He has described with 

 the greatest detail the structure of the component parts of the 

 eye, — especially the retina, where his acute vision enabled him to 

 distinguish the central nerve fibre of the retinophorae , an Observation 

 which his successors were not always able to repeat. The generai course 

 of the other nerve fibres, especially those proceeding from the ganglionic 

 brauch , he was likewise able to follow with wonderful accuracy, 

 when we consider his means and methods of study. 



1 The reference of Garner to the ej^es of Ostrea, in which we are led to as- 

 sume that they are similar to the eyes of Pecten and Siiondylus, is probably due to 

 the fact that he had in mind another species of Pecten. Poli speaks of Pecteii as 

 Ostrea. 



