166 



WM. A. KEPNER AND W. H. TALIAFERRO 



While the ciliated pits are the chief organs of special sense, 

 two other organs of special sense are associated with them. 

 These are the two eyes. One of the unique features of these 

 four organs of special sense is that they are formed by a definite 

 number of cells Only eighteen cells are concerned with the 

 formation of all four organs and this number is always constant. 



Text fig. A Drawingof dorsal aspectof Prorhyuchus applanatus. DG, dorsal 

 ganglion; E, eye; P, pharynx. X 75. 



The eyes appear in the living specimen to lie immediately over 

 the dorsal gangha (text fig. A). Each consists of two cells — 

 an accessory or pigmented cell and a retinular or visual cell. 

 The histological features of this accessory or pigmented cell 

 suggest that it is a modified mesenchymal element. Its mesial 

 cytoplasm and nucleus resemble that of a typical mesenchymal 



