No. 2.] EYES OF POLYCH^TOUS ANNELIDS. 199 



there may be a nucleus. This clear end extends amid the 

 epidermal cells towards the cuticle, while the pigmented end 

 extends inward, and is thought to connect with ganglion cells 

 by means of a nerve process. 



These cells, I judge, may be compared with the retinal cells 

 of the more perfect eyes ; here scattered and not aggregated in 

 such a way that their clear ends could unite to form a large 

 refracting mass. 



III. FORMATION OF EYES. 



The following observations upon the forming and the imma- 

 ture eyes of several polychsetous annelids are unfortunately very 

 unsatisfactory, owing to the lack of sufficient knowledge of the 

 technique necessary to obtain clear results. As far as they 

 extend, however, they aid in completing and in strengthening 

 the general conception arrived at in the study of the adult eye. 



Nereis limbata. 



The eggs are readily fertilized and the larvae reared up to a 

 stage represented in Fig. 55. The four eyes then present are 

 obviously the same in position, and are also the same in struc- 

 ture as those in the young Nereis (Fig. 56) taken in the surface 

 net. These in turn are identical with the eyes of small speci- 

 mens four to six millimetres long (Figs. 5, 21), and thus agree 

 with the adult structure. 



The four larval eyes are, then, converted into the four of 

 the adult. 



In an earlier state (Fig. 54) there are six eye-spots : the 

 posterior four are the permanent ones ; the anterior two, the 

 provisional eyes. 



The study of the formation and early condition of the four 

 posterior eyes of the larva is thus the study of the formation of 

 the adult eyes. 



The first eye-spots appear about the tenth hour after fertiliza- 

 tion, as minute golden red areas, one on each side, between the 

 region of the apical pole and the equatorial ciliated band. The 

 larva is now a spherical gastrula inside the egg-membrane. 



Surface views and macerations of an eye show a limited 

 number of pigment spherules, less than one hundred, irregularly 



