Langdon, Sense-organs of Nereis virens. 71 



///. The Paitcd Cephalic Organs. 



1. The prostomium contains four pairs of special sense- 

 organs, two pairs of eyes in its dorsal surface, one pair of prob- 

 lematical organs in its anterior margin, and a second pair in its 

 posterior margin. There also appears to be present near the 

 base of the inner side of each palp a ciliated area which may 

 prove to be a second pair of anterior cephalic organs. 



2. The evidence at hand goes to prove that the true eyes 

 are derived either from the spiral organs themselves or some 

 more primitive type common to both. It also supports Andrews' 

 suggestion that the lens in the true eyes is composed of the trans- 

 formed tips of the retinal cells — the " Sekretzellen " of Hesse. 



3. The anterior pair of cephalic organs are each composed 

 of a group of large bipolar nerve-cells whose peripheral process, 

 es branch and finally terminate in end-brushes just beneath the 

 cuticula, apparently without any connection with epidermal 

 cells, and whose central processes pass into the brain. 



4. The posterior pair are also each formed of a group of 

 bipolar cells, but these cells have their bodies situated in the 

 brain itself and their peripheral processes terminate among spe- 

 cial epidermal cells which lie on the anterior face of two invag- 

 inations. These invaginations appear to be due to the pull of 

 large muscles — not to the necessities of the sense-organs con- 

 tained in them. 



IV. The Epidermal Anchoring Cells. 



1. There are also found among the common epidermal 

 cells of Nereis virens modified epidermal cells which I have 

 called anchoring cells. 



2. The body of one of these cells does not appear to dif- 

 fer from the bodies of the supporting cells of the epidermis. 



3. From the peripheral end of each anchoring cell a large 

 number of fine processes pass into and are firmly embedded in 

 the inner layer of the cuticula just above the cell. 



4. From the basal end of each cell a small number of 

 slender processes pass to or into the latter. 



5. These anchoring cells apparently serve for the attacli- 

 ment of muscles to the cuticula. 



Zoological Laboratory, University of Michigan, July i, 1899. 



