No. 2.] EYES OF POLYCH^TOUS ANNELIDS. 175 



that tangential sections near the lens represent them as small 

 granular circular areas separated by much intervening matrix 

 (Fig. 26). 



Lens. — The true structure of the refracting central part of the 

 eye is not readily made out, owing to its semi-liquid consistency 

 and liability to undergo artificial changes in various methods of 

 treatment. In sections passing through the visual axis (Fig. 8) 

 this central mass fills all the retinal cups, internal to the rods, 

 and also projects from the cup as a hemispherical mass next the 

 cornea. This projection might be called the lens proper, and 

 the mass inside the cup the vitreous body ; but as both are one, 

 we may speak of the whole mass as the lens. When removed 

 in sea-water, this lens (Fig. 10) is a spheroidal mass harder than 

 glycerine, but having much the same optical appearance. Its 

 larger lobe bears a smaller convex protuberance, the part next 

 the cornea, and in Haller's liquid, shows longitudinal striations 

 such as are seen in sections (Fig. 8). The groove marking off 

 the two lobes of the lens is usually filled by the adhering 

 pigment of the edge of the pupil or edge of the retinal cup. 



When preserved, the lens becomes very hard and refractory, 

 but presents striations that appear to be due to the presence of 

 rods or columns passing from the retinal rods to the cornea. 

 In tangential section such columnar elements may appear as 

 polygonal bodies, shrunk from one another in Fig. 27, with 

 radiating cracks or lines. 



These component elements of the lens sometimes appear as 

 if continuations of the retinal rods, but more often they are 

 irregular and much less numerous. Yet some preparations 

 show in section the continuity of each lens element with a 

 retinal rod, as in Fig. 2, where the line of demarcation between 

 the two is merely a series of vacuoles. This idea, however, 

 must be regarded with much doubt, owing to the great variety 

 of appearances produced in the coagulation of the lens. No 

 nuclei or similar bodies were discovered in the lens, and it is 

 not, as a whole, shut off from the retinal rod by any membrane. 

 Although double stains with haematoxylin and borax-carmine 

 may show a sharp line separating the blue rods from the red 

 lens, this is due to a sudden change in consistency there, not to 

 the presence of a membrane. 



