Eyes of Molluscs and Artliropods. 619 



dark brown or black pigmcnt. The euticula is uot especially developed ; 

 neither eould auy eolorless cells be seen iu the seetious. The similarity 

 of these pigmeuted bands with those on the sipho and mautle edge of 

 the Lamellibrauehiata is at once evident; whether the resemblance Ì8 

 more than a superficial oue, and whether a similarity in fimctionis also 

 p/esent, cau ouly be shown by actual experiment. 



It was Babuchix, who in 1866 ^ published a paper whieh, for 

 aceuracy of Observation , has hardly been excelled by subsequent au- 

 thors faVored with much better means of study. He fully recognized 

 that the retina was composed of groups of pigmented cells, surrounding 

 a central, eolorless one with a large nucleus. He likewise appears to 

 have Seen the intercellular nerve fibres, but erroneously considered 

 them to be foldings in the cell wall. Also the important facts, that the 

 eolorless cells were continuous at their inner ends with a single fibre, 

 while the pigmeuted ones ended in four or five, did not escape bis no- 

 tice. As far as the structure of the retina is concerned, no new acqui- 

 sitions have been made since bis time, except to extend some of his ob- 

 servations to other genera. In the majority of cases, subsequent 

 authors have failed to recognize certain essential facts which he seems 

 to have fully appreciated. 



Hensen failed to find in Pterotrachea the so-called eolorless 

 cells of Babuchin. He distinguished three kinds of cells, one of 

 which, with the large, round nucleus, wasprobably one of the eolorless 

 cells in question, to whose surface, pigment was accidentally attached. 

 One might likewise infer from his description, that the pigment cells 

 were differentiated iuto two rows, as in Area. His observations are not, 

 however, suffici ently complete to determine with certainty whether his 

 rods are formed by the combined products of the pigmented and eol- 

 orless cells , or whether both bodies produce comparatively indepen- 

 dent structures, as in Haliotis. 



The researches of Sdiroth (30), are drawn out to an interminable 

 length, and are accompanied by numerous care fui drawings of histo- 

 logical rubbish , from which he has been unable to culi any new facts 



1 Up to this date the knowledge recorded eoncerning the structure of the 

 retina is of such a rudimentary nature, that it will not be necessary for us to 

 consider it. 



