Eyes of Molluscs and Arthropods. 551 



exist, it seems hardly credible tliat tliey sliould liave so long escaped 

 tbat thorougli examiuation , whicb many less conspicuous and deserv- 

 ing Organs received long ago. 



As early as 1844, Will (5) furnished us witli the first notice of these 

 eyes. He recorded a niimber of fairly accurate Statements of tbeir 

 general appearance and distribution , and, moreover, fully recognized 

 tbe compound nature of tbe faceted eyes , the only oues he appears to 

 bave Seen. His mind seems to bave been strongly impregnated with 

 terms applied to the various parts of tbe Vertebrate eyes , and , ap- 

 parently on tbe supposition tbat , eitber physiologically or morpbologi- 

 cally, equivalent parts were to be found in all eyes, he has applied to 

 tbe Mollusca, terms from Vertebrate anatomy, when a better knowledge 

 of tbe facts shows tbat tbe parts, tbus designated with tbe same names, 

 are widely difierent structures , and can neither morpbologically nor 

 physiologically bave any characteristics in common. A certain »a 

 priori« bias is produced, vs^bich tends to confusion, and from whicb it 

 is difficult to rid the mind. For instance , a simple fold formed by 

 abnormal contractions, he has, in Pinna, called an iris, and has applied 

 tbe same name to the circle of pigment cells surrounding tbe retino- 

 pborae. An ommatidium, he called a simple eye, v^bich, iudeed, 

 could hardly bave been nearer tbe truth, even if he bad bad a much more 

 extended knov^'ledge of tbe matter than be reallydid. The terms tape- 

 tum, choroidea, vitreous body, and iris, appear at all times, 

 and in places where it is difficult, but in fact hardly necessary, to de- 

 termine to just what structures these terms were applied. His imagina- 

 tion carried bim so far tbat he has described minutely eyes of certain 

 genera of Mollusca, tbat it has required nearly forty years to prove do 

 not exist, unless indeed be examined other species than those at tbe 

 command of subsequent workers. I repeat it is a remarkable tbing tbat 

 this description, as accurate in Observation as it was fantastic in 

 conception, should not bave drawn tbe attention to this subject tbat 

 its importance deserved, or provided us with a more detailed description 

 of one of tbe most remarkable Systems of eyes — if I may use tbe 

 term — at present known to exist in tbe whole animai kiugdom. 



Subsequent authors seem to bave been satisfied with Will's des- 

 cription, and deemed further knowledge on tbe subject unnecessary. 

 This was the case with Siebold (7), Schmidt (17), Sharp (18) 

 and Gegenbaur (14) . Tbe latter erroneously speaks of the eyes of Area 

 and Pectunculus as being borne upon special eye stalks. He also says, 

 referring to tbe eyes ^iPecten^ Spondylus^ and other Lamellibrancbiata, 



