ON THE EYES OF POLYCH^TOUS ANNELIDS. 



E. A. ANDREWS. 

 I. INTRODUCTION. 



The most complete and satisfactory description of an annelid 

 eye is that given by Greeff in his monograph of the Alciopidae, 

 in 1877 (1). 



Subsequent observations in the same group have, in the main, 

 but confirmed that account and added but little to it. The 

 observations made from time to time upon the eyes in other 

 families have been frequently but incidental to descriptions of 

 the entire anatomy, yet they present great advances over the 

 superficial knowledge utilized in the older systematic works. 

 Some reference to these scattered observations will be made in 

 connection with the particular forms studied in the present 

 paper. At present we will notice only the two chief concep- 

 tions of the annelid eye to be found in the better known com- 

 parative works of Graber (2) and Carri^re (3). 



The former studied the eyes of many annelids, and saw in 

 them grounds for comparison with the eyes of arthropods. 



The latter observed several annelid eyes, and regarded them 

 as similar to the gasteropod eye. 



The annelid eye in Graber's conception is a duplex structure : 

 an open, pigmented cup, the retina, closely associated with the 

 brain, and cut off from the epidermis ; a mass of epidermal cells 

 filling the cup as a refracting accessory part of the eye, and not 

 connected with the retina. The retina is composed of peculiar 

 elements having at least two nuclei in each. 



In Carri^re's estimation, the annelid eye is a closed sac com- 

 posed of the retina and the inner cornea, filled by a refracting 

 non-cellular mass that is not connected with the epidermis. 

 The retina is a single layer of cells in which pigmented sensory 

 cells alternate with clear, secretory cells. 



Other authors have referred the eye to one or the other of 

 these fundamentally different interpretations. 



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