Langdon, Sense-organs of Nereis virens. 55 



consider it impossible that these spiral organs should be glands 

 of any kind whatever. 



The phosphorescent organs of some invertebrates have 

 practically the same structure as simple ocular organs. The 

 two have often been mistaken for one another and are supposed 

 to be derived from the same primitive organ. From its struc- 

 ture the spiral organ might belong to either class. 



I have not found in the literature any record of phosphor- 

 escence in Nereis. Moreover, as much as I have worked with 

 the living spiral organs, crushing them in sea-water, exposing 

 them to the air, and watching them for hours under the micro- 

 scope, I have never seen any trace of luminosity. Experimen- 

 ters on phosphorescent organs have found that an organ be- 

 comes luminous when removed and crushed in water, even, ac- 

 cording to Vallentin and Cunningham ('88), in the day time. 

 It therefore seems to me, that the possibility of these organs 

 functioning as phosphorescent organs can be set aside until some 

 direct proof of this function is brought forward. 



This leaves the ocular, or at least the light perceptive, 

 function as the only probable one. 



The simplest form of ocular organs among invertebrates 

 is that described by Hesse ('95) for the Lumbricidae and closely 

 related forms. Each organ consists of but a single cell. Each 

 cell possesses a basal process which is probably a nerve fiber, a 

 nucleus in its basal end, and an irregularly prolonged peripheral 

 part which contains a refractive body. If one imagined this 

 peripheral process drawn out into a longer process with the re- 

 fractive body confined to its extreme tip and changed in shape 

 by the pressure of surrounding parts, one would have an indi- 

 vidual cell of the spiral organ of Nereis virens. 



Sharp ('84) has described in Solen ocular organs each of 

 which consists of a group of cells arranged around a central in- 

 vagination. These cells have clear peripheral ends, pigmented 

 middle portions, and a basal part containing a nucleus. Except 

 for the pigment, this would correspond in general with the 

 spiral organs. 



Andrews ('91) has studied the "branchial eyes" of several 



